Hello! Yes I have the same problem every once and a while, especially when we're doing it doggy style. And it seems like it could be a number of things. Last night though, it hurt so bad we had to stop because i started crying. Usually I can deal w/it but holy crap. I thought something was going to be ripped out of my stomach it hurt so bad! I had read a few people saying that this can happen if you have a tilted uterus....which I do have apparently. At least that's what the gyno said...it took her forever to find it when I went! (how embarrassing that was!) I don't think it's really anything to worry about! Unless it happens EVERY TIME you have sex, definitely go see a dr asap! But if you're like me and it's maybe once every few months (if that), odds are he's just a little too big and hitting your cervix or maybe you have a tilted uterus! I don't know if it's the same for you, but I don't get this pain every single time we are in any kind of deep penetrating position...it's only sometimes.
I can also say....that when it's close to my period ending or beginning, I get really bad cramps....the other night it happened to me actually now that I think about it! It hurt so bad after wards that I starting digging my nails into my boyfriends arm when we were laying there because it hurt so bad! But like I said, that's only when it's close to my period ending or beginning. Is your pain every time you have sex?