Possibly TMI. Sex question!

Okay so I'm having this problem. Whenever my boyfriend and I have sex, we switch from position to position after a while. Sometimes, I'll be in a position where he hits it at just the right spot and it feels good of course, but it will also feel a bit painful. Why is that? Like within the pleasure I'll feel a bit of uncomfortableness. Why does this happen? I'm pregnant but this has been happening since before I was pregnant. I want to be able to enjoy those great positions without stopping because it feels a little weird . I don't know if that makes sense. Nice responses only please!!