Sex making me bleed....

So to start off I'm on birth control and I was almost to the end of my pack two weeks ago. Well me and my boyfriend had sex and the next day I started my period. Idk what triggered it. But I had it for 10 days which is odd but it finally stopped. Well we had sex a few times the past couple of days and all of the sudden  I'm bleeding again. Does anyone have any idea why? I've never gone through this. I'm 21 by the way.