Weird cramp during sex? anyone else?

The other day when I was having sex (him on top) I felt a dull pain in my lower uterus. it was ignorable, so I let it go, but a few days later it happened again. This time it was m u c h worse. a very, very sharp pain that lasted for a few hours (it hurt to walk or sit down sometimes) deep in my uterus on the right side. I dont know much about the muscles in that area. Could it have been a muscle cramp or spasm? I felt it a few times after but it was only a sharp pinch for a few seconds. anyone else ever experience this? I was wondering if it could be a sign of pregnancy or something serious... or if I should just let it go, ignore it.