I had an arterial problem for a couple of years, which reduced blood supply to my heart and brain and depleted B vitamins from my nerves (to keep the heart in good repair). Although there is some vagueness as to the mechanisms, this made me forgetful, slow, and easily overwhelmed. In short I felt like I was stupid compared to what I was used to, and I was.
It was frightening at first because I knew something wasn't right but didn't know what, and very worrying for my career because I was simply not very good any more.
However, once I got used to it and resigned myself, it was great.