I'm not a doctor or anything, but here are my guesses.
a) Dehydrated. You exercised and then went to bed. Did you have the time to adequately re-hydrate?
b) Are you eating enough? If you are exercising then going straight to bed, I'm guessing you didn't eat anything in-between, or eat right before you exercised. That a long time for your body not to be refueled.