What Causes Irregular Brown Discharge Between Periods?

I was on nuva ring for about 4 years. I was going to stop taking birth control all together but then I was having a brownish discharge and irregular periods. I went and had a yearly checkup and my pap smear and STD tests all came back normal. I got on the pill and continued to spot the brownish discharge sometimes stopping for a few days then starting again. Almost 1 year and 3 different pills later I still will have this brownish discharge on and off. But I still have normal periods. Could this possibly be a hormonal imbalance?