Dark/light brown vaginal discharge?

Emily
I've been having oddly colored discharge for a few weeks now. At first I thought it was just left over from my period, but it's been many many weeks since I ended, and still have brown discharge. It happens very commonly too. I am on birth control, but it was never like this in my previous month of taking it. 
Does anyone know the reason for this or a way to get back to normal? Thanks for any help