insides hurting SO during sex...?

I'm 20 weeks pregnant and my SO said when we had sex it felt like two bones (or he said it could have been really dense muscle idk) inside me were almost pinching/hitting his penis... he said my hips must have moved around and he doesnt want to have sex now because it was painful. What the hell could this be? I'm worried, and awfully confused. He knows what a cervix is, and it wasnt that.