Nearly a month ago I had "unprotected sex". It was my first time but we barely did it because I was hurting a lot when trying to go deeper. I felt like I still didn't lose my virginity. 2 days later I felt nauseous and had cramps, then threw up and felt fine. Exactly a week after "doing it" i experienced some bleeding followed by dark red/brown clumps. In fact I used a pad but it only lasted a few hours. I am worried that I could be pregnant because my period is a week late but I have quite an irregular cycle.What can I do to prevent it. HELP!