Pain during sex

I've been on the depo shot for 9 months (I just got my third shot last week) and for the 3 weeks or so I've had pain whenever my boyfriend and I try to have sex from behind. It's not because I'm not loose enough. I still get the pain even if we've already had sex in a different position (switching positions from missionary to behind). The pain starts as soon as he starts going in and gradually fades after a minute or so of just having it in there (no thrusting) I can't figure out what the pain is coming from. Any thoughts or advice?