What Causes Brownish Discharge During Menstruation?

24 year old female. I have had irregular periods for 11 years. they usually come 2-6mos apart. my gyna has not found any clinical abnormality fir this ocurrance. I recently had a pap smear and was tested for sti/stds. I recieved a letter that said neg for sti/std but abnormal pap. I have had a brown vaginal discharge for the past 4 days. i usually get one day of this discharge the day before my period starts, but never for 4 days with no blood. I have a gyno appt to follow up with the abnormal pap in a week. would this be a symptom of cervical.uterine cancer..should i press to get in to the doctor earlier.