Cramping after orgasm?

I'm 19 y.o., and I've been on the birth control pill for 3 years now. Just recently, after clitoral orgasm, I will get intense period-like cramps that last about 20 minutes. I've been masturbating for quite a few years now and haven't had any recent changes in my medical history.  I've read a few things online about this happening in early pregnancy and this is freaking me out because I hope I am not pregnant!!  I am also a week away from my period.  Does this happen to anyone else, and if so, do you know the cause? Thanks!!