Bleeding halfway through my pack of Bc

I'm 18, and just started jesmeil, I'm on my second month on birth control for the first time ever. I started spottinf a few days ago, and I wasn't super worried. Right now it's almost like I'm on the last day of my period, dark brown bleeding and an almost like pasty shedding. I've always assumed it was uterine lining, but I'm getting it now. Do I need to be worried or is it just breakthrough bleeding?