Sex has started to hurt/burn, etc

Ever since I got on Nexplanon, sex has changed. My libido is really low and affecting our sex life. I think part of why it lowered is because sex started to hurt and so I stopped wanting it as much. I might make an appointment for the doctor soon to find out why it's been hurting. When he first goes in, it burns a bit and is super painful. After he goes all the way in it feels good but burns a little bit. What could this be? I'm 14 and just started birth control. Should I make a doctor's apt and see my ob-gyn? Also, slightly off topic I've been on my period for 11 days now (not spotting, bleeding a lot). I assumed this is because of starting birth control.

Anyways, what could the pain be?