What Causes Spotting, Stomach Cramps And Nausea After Having Unprotected Sex?

Had 3 periods in one month. I m 21. MY period have been very regular since I was 16, normal 29 +1/-1, 5 days with a medium flow. Never been on birth control. January 31st I got my period, on time, and normal, then I had sex February 7th. Then the 11th I had sex again, then I started spotting brown discharge the 12th, then that night I got really sick stayed up throwing up, horrible cramps, chills, nausea. The next day (13th) I went to the hospital, and they tested for stds, pregnany & pregnancy all all came back negative. Then sent me home just prescribing pills for nausea, which I didn t take. Later that day, I had bright red vaginal medium bleeding, minegraine, throwing up from nausea. I bled for 3 more days then it stopped like a normal period. So my next scheduled period was suppose to be the 28th of February but didn t start till March 8th (10 days late) with cramps and light period(for the last 3 days). I haven t had any changes that would cause this. Any advice? P.s. Me and my finance don t use any birth control methods.