What Causes A Vaginal Discharge Instead Of A Period?

Hello, I just got some watery brown vaginal discharge. I wanted to know what the implications of this could be? I took a pregnancy test about two hours ago and it came across as negative. I'm supposed to be have gotten my period yesterday. I had unprotected sex for about ten minutes a week or so ago, but my partner withdrew out of fear of impregnating me. Additionally, two weeks before I used an emergency contraceptive. Thank you for your help!