Bleed after sex!

I have a boyfriend that I've been with for 6 months and every time we have sex I bleed after, but it's only spotting it's not very much but it's never happened before with anyone else it only started when we started dating and being sexually active. I know I'm not pregnant I'm on birth control and had my period recently. I don't know why it happens and it's kinda emberassing sometimes. Anyone have any ideas why this could happen?