I have had heavy dark brown discharge for about a week now. i am 4 days away from my period and i am on birth control. i have had some cramping.cause?

Normal: Missing birth control pills or taking them at different times every day can cause breakthrough bleeding. Sometimes, you may have this episode for no reason at all. Just poor absorption of the pill or some associated illness like the Flu or taking antibiotics simultaneously can also do it. So, just wait and see if it normalizes in the next cycle.