Pain before and after sex.

My bf and I don't have sex too often. Once every 2 weeks or so. Before we started dating, if I was in a relationship I would have sex a couple times a week. But he has a lower sex drive than I'm used to. Which is fine, but I feel like when there is a long time between sex sessions I feel more pain in the beginning and after (during I don't really notice) I feel like I get tighter the longer it's been, but I've heard that isn't possible. I don't hurt on the inside, I just feel like the opening of my vagina feels kinda stretched. Does anyone else who doesn't have sex all that often have that problem? And is the issue because it's been awhile, or something else?