What Causes Mid Cycle Bleeding After Intercourse?

I'm sure you hear these questions all the time, but here it is: What could be the causes of the blood coming out of my vagina? Sometimes it's light red and sometimes it's more brownish in color. I'm not supposed to get my period for another 8 days, but this spotting has been going on for 2 weeks. I am sexually active, and I have been for 6 months, but I've never had this problem before. I don't necessarily hurt, but I have been more tired lately.