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.