What Causes Dark Brown Vaginal Discharge While Recovering From UTI?

I have a dark brown odorless discharge. It has been on and off since my period. I am on OrthoTriCyclen Lo. Some days it is barely there and others it creeps up on me and i should have been wearing a pantyliner. I've been having unprotected sex and I just got over a UTI. I have no idea what it could be. When i was diagnosed with the UTI I also got a chlamydia test, it came back negative. I am so unsure as to what this could be. I just want to stop worrying.