I've been practicing bikram yoga for a year and a half, and after class I am always a mess.
About an hour later I'll have a horrible migraine along with fatigue, and light-headedness. I know that it's recommended to drink a lot of water after class since you've lost so much, but that doesn't seem to help no matter how much/how little I drink.
I think the problem is with electrolytes. I try to eat something like dates after class, or vega bars/larabars. I've also tried combining agave nectar/lemon juice/sea salt into a big water bottle, but it still doesn't work. Maybe I'm getting the ratios wrong? It usually helps if I have a small nap soon afterwards, but that isn't always practical if I have class or work later on in the day.