What Causes Continuous Black/brown Discharge While Taking Birth Control Pills?

hello ive been breastfeeding my daughter for 11 months and im on the mini pill. ive only had my peiods once when i started the mini pill. i started my periods in june i was bleeding for a week but since then im having black/ brown discharge. Its not clearing sometimes i have blood but ive always got the black/brown discharge every time i go toilet. Im really worried please help