I learned from various people I've talked to who have had actual health problems that doctors often fail to figure things out (Examples: They do a pile of tests that come out negative and shrug it off or throw pills at it and never get down to the root cause.) and even fail to give useful referrals, so if that's why you're asking, I can grok why you're looking for answers here.

I find that if I just go to any old doctor, they have no idea what is wrong. The only way I've ever been helped is to find someone who specializes in my set of symptoms and has lots of experience with it. The absolute best thing to do would be to find somebody who has had the same problem and has seen it resolved by a doctor who specializes in those symptoms, and get a referral to THEIR doctor. (That, more specifically, was what I did.) A group dedicated to your problems might be a good place to ask for a useful referral.

Barring that, a doctor or GP who has a special interest in your group of symptoms is likely to be more useful than an arbitrary general practitioner. GPs have to remember tens of thousands of symptoms and match them to tens of thousands of conditions - not a very reasonable thing to expect of a human brain when it comes down to it. That's a key reason I think the people I know are so frequently having problems with getting a useful diagnosis. (Note: www.diagnose-me.com claims to help counter this problem. It's a program that takes a 1000 symptom questionnaire and searches a bunch of medical data to give you suggestions for what might be wrong, and they have doctors on staff to review the results as well. I've tried it - it's inexpensive and marvelously detailed. Not all their info is accurate, but if your expectation is simply to get ideas to have checked by your doctor, this is likely to give you some.)

I'm not a doctor, but here's an idea you can take to a medical professional to get evaluated for if interested:

Your digestive system and immune system seem involved. It occurs to me to mention that one can have an infection in their digestive system without realizing it. Aside from your immune symptom (allergies), and digestive-immune link (I read that the majority of your immune system is located in your digestive system) there may be another link: I read somewhere that the immune system outputs substances that cause aches and pains when fighting infections (if I am not mistaken, one specific substance that causes this is tumor necrosis factor-alpha).

Consider getting tested for microbial imbalances in your digestive system. Candida albicans, a yeast, is supposed to be a common cause of dysbiosis and might be especially useful to research.

Before killing bugs in your digestive system, ask the doctor if doing dietary modification before taking the anti-microbials would be a good idea. Helping them multiply by giving them the foods they prefer while also killing them off can result in a prolonged Herxheimer reaction. This essentially results in feeling 10x worse for however long.

Let me know if that helps.

I've had a possibly similar problem with my right leg. It's almost entirely cleared up now, presumably as the result of high quality massage. I'll ask the person I go to what her background is if you're interested.

Could be some variety of auto-immune