What Causes Bleeding After Sex?

Hi, I had sex with my boyfriend two days ago. It was unprotected-- he didn't have a condom and I'm not on the pill. It was kind of a quickie and he didn't cum or pre-cum. A little while afterwards I went to use the bathroom and there was blood on my panties. I'm not a virgin and this has never happened before...I thought it was my period but that night I was only spotting and the next day I spotted a little, too. What could it be?