Have any past sexual trauma you're shouldering? That can be another reason.
Another is that orgasm is a really powerful thing, chemically speaking. So, crying, laughing, even wanting to shout at the top of your lungs really can happen and be absolutely nothing to worry about, I promise.
It may well be you're just having profoundly great sex.
Editor and Founder, Scarleteen
My epitaph should read: "She worked herself into this ground."
-- Kay Bailey Hutchinson