Super long period, WHY!?

Can someone please tell me why I'm on the 10th day of my period!? I'm 17, turning 18 in December. I just started birth control for the first time and I'm 3 weeks in. My period was 2 months late but it came when I started the BC, but now it won't stop! I wanna say its because the BC is just trying to regulate it I guess? But I'm really freaked out. Also, I have had history with anemia so maybe that has something to do with it too.