Persistent Dark Brown Discharge, Lower Abdominal Pains. Sexually Active, Using No Birth Control. Reason?

over the past week or so, I've been experiencing dark brown discharge on my underwear/pad liner and initially it didn't bother me but it's been persisting for quite awhile now so I'm concerned. I've had brown discharge last time between periods before but they were never as long as this. and everytime after urination, when I wipe, I would see brown mucus like discharge and sometimes I experience lower abdominal pain (I'm unsure if my mind is playing tricks with me or it's true). I'm 22 and I'm sexually active with no birth control. me and my bf are each other's first and we have only one partner so I would rule out STD. and he always pulls out before ejaculating so I would rule out pregnancy too. Please advise, thank you!