Sex with SO

Me & my SO do not have sex very often, but tonight we had sex & he was talking dirty to me saying that he wanted to hit it from behind after I climaxed, I have to be on top to do that. So before I could even climax or really even get his penis in, he orgasims. He stopped, looked at me & said "ever since you got pregnant it feels so much different in there" lol. Does anyone know what he is talking about? I asked him to explain it to me but he doesn't know.