Pain after sex

My current partner and I have a lot of sex. We only see each other once or twice a week, but easily have sex 2 or 3 times a day when we do get to be together. I think too much sex is causing some pain near my the opening of my vagina. If I had to describe where, it's on the very back rim of the vaginal opening. It burns if pee touches right there and it's very painful to have sex in certain positions. If I had to guess, I'd say there's a small tear down there. This is becoming a recurrent problem though when we have a lot of sex. Anyone else have this issue before or have any ideas of what to do about it? We don't use lube because I feel like there's enough natural lubrication, but maybe we should? Maybe too much friction? I don't know, but it hurts and I'd like to keep having lots of sex, lol.