What Causes Brown Discharge And Cramps?

cycle questions I am a 25 year old female that weighs 156 lbs. my question is that my cycle came on April 8th and ended on 15th. my boyfriend and i was intimate on the 15th and did not use protection. my fertile days was like 17,18,19,20,21.. we was intimate again on the 22nd which was my obvulation day but we used protection on that day. through out that week i been having very sharp pains under my stomach and has dark brown discharge for two days.. the other days has been clear and very moist. what is the cause