Painful sex?

For some background info, I'm 18 and not pregnant. This hasn't ever happened before.

As of a few months ago, sex has become extremely painful. The right side stings and it's unbearable. It hurts most in missionary. Even if we keep going, the pain only persists and rarely decreases (and when it does, it's not by much). The only times it hasn't really hurt is when I'm extremely turned on and it's from behind. It always hurts at least a moderate amount. The majority of the time, we have to stop immediately. I'm usually wet enough, and even then, most of the time we use a really good working lube. Also, fingering doesn't necessarily hurt as much, if at all sometimes. Maybe that's just because they're not as big, but I thought I'd note it. Any tips?