What Could Be Causing Brown Vaginal Discharge With Abdominal Pain?

about three days ago i started getting a dark brown, bloody, mucous discharge and at times it looked like it had stringy stuff in it, since then it has gone away but i have mild abdominal pain. the abdominal pain comes and goes and isn't really localized to one specific spot. i'm really nervous what this could be. i am sexually active and had unprotected sex over two weeks ago but took plan b immediately afterwards. the first day of my last period i believe was around july 1. so i should be getting my period this coming weekend or early next week. in the past i have had clear/whitish discharge before my period or even some brown discharge immediately (day or two) before my period.