Sex cramps?

Sometime while having sex w/ my bf and I'm riding him I get s really sharp pain about 2inches from my belly button (left or right. It varies) and about 3-4 inches. It's a very pinpointed pain. Not like a cramp after eating and exercising. It hurts so bad we have to stop and I have to lie down until it goes away. It happens maybe once or twice every couple of weeks. Help