Heavy spotting for 8 days mid-cycle.

Hello, I'm a 20 year old female and I'm 5'9" about 125 lbs. My period ended on March 3rd and on March 7th my bf and I had unprotected sex and he pulled out well before he climaxed. We had unprotected sex again on March 9th but we finished before he finished at all due to more-than-usual sensitivity in my vagina, almost like pain. Well on March 14th I started spotting pretty heavy and that night I had some of the worst pain/cramps in my abdomen I've ever had. The next day the pain subsides slightly and on March 16th, the pain left altogether. However, it's March 22nd and I am still bleeding, heavy enough to keep using tampons. This has never once happened to me before so I'm not sure what it is. The only symptoms I've had since pain is bleeding. I have been eating very little and working out the weeks leading up to March 13th because I was trying to lose as much weight as possible before Spring Break. Can you maybe borrow down possible causes for this?