restarting my period???

Okay so my period ended 4 days ago and i had some light spottingg in between but around last night the light spotting returned and now its basically a light period but its like pitch black?? It seems like spottig but its a lheavier than usual spottig and lighter than a period? I dont know if this is because i started birth control a week ago and this is a side affect??! Can some one please explain or help me out ?