help??

Okay so about a month ago and last weekend I had unprotected sex, a few days later brown discharge starts to come out. I start my period but I have no cramps, it's still a brown color and it's been happening for almost a week? This isn't normal for me. But it's not coming out like a normal period? It's just staying in my vagina. I know it's there cause when I use the bathroom and I wipe.. there is a blood. In my YEARS of having my period, this hasn't happened before?  Should I be concerned? I'm on birth control and I missed a few days last month.. could that be it?