You are very young and I don't want to worry you, but you may have endometriosis. I am now 31 and have had a complete hysterectomy because of this. I, too was having cramps after an orgasm, but I also was having other symptoms. Can you tell me if you are having any other symptoms, like pain during intercourse, or pain in the uterus or ovary area, or fatigue? Hopefully you don't have any of these other symptoms, maybe it is something that is like Mag94 suggested. Hope this helps some. I had endometriosis since I was 16, so I know it can happen at a very young age.
It could be just simply your uterus contracting in an ###### like way (some women experience uteral #######). Not everyone gets pleasure out of this, and you're one of those people, I'd say. But check with your doctor, just to make sure there's nothing wrong.