I just found this thread. I've been suffering with severe fatigue for a while (years). Recently, I was ill for a few weeks. Intense exercise has mostly made me ill for days after. Upon reading all this and having worked out yesterday and feeling quite ill today, I took an antihistamine as I had some already. Not sure if placebo, but within 30 minutes my head feels clearer than it has in days! All I can say is, cheap and safe way to test!
Assuming that you are practicing hard and mean side effects like flimsiness, sickness, and dazedness, I can consider two reasons: drying out and/or hypoglycemia, in all probability the previous. Have a go at hydrating amid or quickly in the wake of working out. Immaculate water is fine; I do well with Gatorade (50 calories, no caffeine) or a proportional. This replaces the salts you free when you sweat and the low measure of sugars (15g) holds off weakness. Bewared of games/caffeinated drinks that can have more sugar and caffeine. You needn't bother with the calories and your heart needn't bother with the jolt.
I've having the same problem after light yoga practises. I'm now even thinking of going back to swimming and weightlifting.
You might want to get an hiv test asap. I read that after working out or just running for a little bit people with hiv get sick (flu like symptons) right away.
Maybe it's exercise induced asthma?
You might have something called Wegener's a very hard thing to diagnose. It has flu like symptoms and can be diagnosed by blood tests. I suggest you get looked at by a Rheumatologist who has some experience with this disease. It will kill you if untreated.