Congratulations on fixing your problem!
I have an alternative diagnosis though: Maybe it was gout? That is, sodium urate crystals causing inflammation and pain in or around the joints. Toe involvement would very much suggest that, also the fact that it improved relatively quickly with a change of diet. Hips are not the most characteristic complaint in gout but I know two guys in real life who are on low purine diets and one of them is doing it for his hips. Doctors also found clinical indications of wear in his hips when he was around your age, but almost 2 decades later he is still doing great amounts of physical work, just the kind of which is known to be very hard on the knees. He's not audibly complaining AFAIK. What I want to say with this is that a diagnosis of osteoarthritis is not necessarily the end of the world. It can be that too, of course.
And I also know one case like this in fiction! There's a Danish-Swedish television drama where one of the characters has scrotal pain and a mortal fear of testicular cancer. He goes to the doctor and is told to lay off the beans because of gout in the hips. Just something I'm primed to notice in TV playing in the background.
Did you eat a lot of beans and lentils before the switch to more animal protein? Legumes have a good amount of purines in them. You mentioned fish and maybe opportunistic meat eating too? Was that small fish with skins, such as herrings, sardines or whitefish? Did you eat organ meats? Liver for the B12?
You mentioned one-legged squats. Ballet dancers do all kinds of high torque one-legged movements hinged at the hip and they often have snapping hips and hip pain; this is maybe because of the tendons are overgrown or just irritated and swollen from having to move in inconventional paths. If the pain returns, try some symmetric lifting for a while.
You mentioned pain after sitting in a weird position. Was that bent forward? That along with lunges making it worse sounds like it's the iliopsoas and its tendons in the lower end. Also known as hip flexor or tenderloin. When it's feeling sore avoid rapid and jerking movements like spurts and kicking activities like soccer. Wikipedia articles on snapping hip mention that symptoms in the iliopsoas area, as opposed to the iliotibial band around the greater trochanter, are assosiated with problems with the hip joint itself. I guess gout can be the sole cause without any irreversible damage in the joint. In any case remember that there are legs blown off in Ukraine all the time; many babies in Africa don't get their hip dysplasia fixed and live a life void of prospects and full of pain, of which you've only had a small taste; there's more to life than sports and running etc.
Disclaimer: I'm not a medical expert. I read professional magazines now and then.
I have great hopes for the new generation of doctors. I was pleasantly surprised when one said something to the effect of: "You did not evolve to be that frail. Don't blame yourself." I find this a great improvement from the doctors of old who had to posit all kinds of weird theories just to avoid saying "we don't know". Stuff like the theory of retrograde menstruation as the cause for endometriosis, or genetic etiology for common conditions like obesity, literally adds insult to injury: You are the cause; it's something you did; it's something you <i>are</i>. It's all right to entertain different possibilities when you're doing research, but why bother patients with some unproven trivia that does not help in the least?
I claim that the best practice when encountering common ailments with unknown etiology is to tell the patient that the most probable ultimate cause for the condition is a germ of some sort (Cochran&Ewald). The next most probable cause is some other environmental factor, the patient himself is most likely faultless in this regard. It's truthful and kind of soothing too.
Or, "How I became even more disappointed in the medical practice in the USA than I already was".
I'm getting older - half a century now - and have recently been having health problems. It started with soreness in my right hip about four years ago, gradually getting worse and worse over time. At one point I damaged it by sitting in a funny position for too long; later on, I did what I thought was a very normal set of lunges in the gym that made me almost unable to walk for months afterward.
Then, about a year ago, I started having issues with my feet. At first, they were a little sore. They got worse and worse, eventually getting to the point where I again couldn't walk comfortably. They just would not heal, no matter how much time off I gave them.
In the past three years, I've seen half a dozen doctors. Hypotheses ranged from early onset arthritis, to joint damage, to tendon damage, to disc compression in my back. Physical therapy helped a small amount, but the gains tapered off quickly and did not last. I was making no progress, and had simply restricted activity in my life to the point where things were tolerable. None of the doctors had any idea what was going on, and every visit was hundreds of dollars to be told things I already knew. One doctor prescribed exercises for the elderly to build muscle tone, without even bothering to ask about my current exercise regime or current state of fitness. (At the time I could do sets of 40 push-ups and one-legged squats to the floor and back.) To say that I was frustrated is an understatement.
A bit over six months ago, I began to wonder if perhaps this was a side effect of the low dose statin I had started taking. The problems started prior to my taking the statin, but I found reports of statin side effects that looked vaguely similar to my issues. Perhaps I had fouled my cholesterol metabolism?
I concocted an experiment to stop taking the statin for a few weeks, and eat a ton of eggs to try to get more cholesterol in my system. I didn't know at the time that dietary cholesterol makes very little contribution to total cholesterol levels, but that turned out to be a good thing: I saw what seemed to be noticeable improvement. The next couple of months I learned about cholesterol metabolism and ran more experiments, eventually narrowing things down to the point that I knew statin use was not responsible. The eggs were the active ingredient.
But how does that make any sense?
At some point it occurred to me that eggs also contain protein, that bodybuilders supplement protein in order to build muscle, and that vegetarians could get protein deficiency. With ten minutes of investigation, I discovered that my preferred diet is pretty close close to vegetarian, and had slowly been shifting to become more vegetarian over time. Did I have a protein deficiency?
Lacking any other options, I used the power of "moar dakka" and tripled the amount of animal protein in my diet. Since then, both of my long-term multi-year health problems have gone away, all in the space of the last three months. I can hike for miles again, I can lift heavy weights, and I heal after taking damage. It's amazing.
What has not healed is my opinion of medical practice in the USA. How on earth could something like this go undiagnosed for years? How could something as fundamental as protein deficiency not be a standard, ordinary thing we test for? How could half a dozen smart, capable, and highly trained doctors miss something like this repeatedly? How is it that the best modern medicine can do is say "go to physical therapy and hope it gets better"?
To say that I am disappointed quite undersells my change in opinion. I previously took what doctors said with a grain of salt but largely believed them. Now, I take what doctors say and run it through the same sanity check filters I use when surfing random web sites. Diagnoses and recommendations from my doctors are now strictly advisory: it's clear that the person closest to my problems is me, and it's clear that doctors know at best only fractionally more about what's going on in my body than I do. And further, I have more incentive to figure it out, more time I can spend digging into it, and I can run experiments to collect data instead of guessing.
(If only we had a dedicated fraction of the population that were trained experts we could rely on to help us understand, fix, and manage problems with our bodies!)
This is not how an effective society runs its medical system. This is not how medicine needs to be, if it's going to be effective. This is Moloch in action; this is institutional failure. I don't know how to fix it, but at least I know enough now to begin protecting myself from it.
I just wish I didn't have to.