Weird week before period?

My period happens on the 7th of every month. My boyfriend and i only have sex on the weekends because we work and go to classes and have busy weekdays. But on weekends my boyfriend have sex at most 3 times a day. But lately the weekend before my period when my boyfriend and I have sex I am bleeding. I use tampons and its like gooey and small amounts of brown, but I'm not dripping. Not till my period. Is it because of the sex? Or is it because my period is lasting longer now?