It burned after my first time having unprotected sex.

I'm 16 years old and I'm on the Depo birth control shot. I've never had unprotected sex before. Me and my boyfriend recently decided to try it. After I had a burning in my vagina after he came.. He was a virgin so the chances of a STI are low. But i wasn't... I've had one other sex partner but I've never had unprotected sex with him. I'm wondering why this could have happened? Should I be worried.. could it possibly been friction? I'm just curious but it hurt so bad i could barely even just sit down.. Could it be possibly that I have something? Please help. %-)