Period type pains are probably exactly that - period pains, in that they are caused by the constant contraction and relaxation of the uterine wall which you need to do in order to expel a lot of liquid (be it blood or ejaculate) from the body.

A 'normal' orgasm is pretty tiring as it is (if you do it right ) so one of these must be even more tiring than that. The dehydration probably doesn't help. You could try drinking water before or after sex to prevent this feeling. I know I always feel thirsty afterwards and I don't lose all that much fluid during and am usually fairly efficient at water control in that I rarely get dehydrated.