Stings during sex

This is happening frequently but now when my bf and I have sex my vagina starts to stings. So much, especially near the entrance. At first it wont hurt then as we keep going it starts to sting a lot. Like a burn, when he pulls out and back in is when it hurts the most because it touches the entrance. It doenst make me feel good and starts to effect our sexual relationship.What could it be? 
Btw i'm on birth control so we dont use condoms.