Okay, so my boyfriend and I had unprotected sex on feb. 9 and I had my last period Jan. 21 so I wasn't ovulating when we had unprotected sex. But still, we got scared and I took an emergency contraceptive. On feb. 18 I started bleeding red blood and then on the 20th it turned to brown blood. It was all pretty light but heavier than spotting. It continued until the 24th and then it stopped but on the next day I got red blood in the morning and then it turned brown and eventually stopped that day. What is causing this?