pain during sex

So my boyfriend and I have been having sex for awhile now, but the other day we started and I noticed I was feeling really uncomfortable, and I couldn't figure out what it was at first. My vagina wasn't hurting or anything, and then I realized it was hurting my ass. I don't know has anyone ever noticed that before? Like his dick was in such a way that it was stretching me? I don't know it was very weird.