Brown/black discharge

I have had a dark brown/red/black discharge for about two weeks and it is only enough to change my pad once a day. It started a couple days before my period, stopped for a day, then my period was red when it started, like normal. I changed birth control to the one where you just continue taking it for 3 months so you only have your period every three months. I started it a day after my period started, which was what the pharmacist told me to do because they had to wait for the prescription. The day after I started to take the new pack was when it started to get dark again and I don't know if I need to be worried about it?