Period tmi

My period is quite regular, and has been since it started. I got mine like normal this month and I'm on day 5/6 but it has gone from the normal period blood to a more stringy, brown goopy texture. I did have sex (with a condom) last night, so my thought is that it is from the increased oxygen making it darker in color, but I'm not sure. When I was in high school, it used to get to this point every month but when I got to college it stopped (this is my second year). I don't think I'm pregnant because this used to be common for me and I didn't start having sex until this past summer, but I was wondering if anyone had an idea as to why it does this sometimes?