Bleeding after having sex?!

I was wondering most of the times when i have sex with out really being excited, the next day or the same day i start bleeding, but usually is brown red and usually not heavy nor is it really light, and usually lasts for about a week...but it goes from the first day a lilttel then more, and the little until it ends, and this usually happens a week after my period ends.I also forgot to mention im irregular, being that my period usually either comes on time or sometimes it comes three days or a week later. So i was wondering why this happens? %-)