What Causes Brownish Discharge After Menstrual Cycle?

I'm at 17 year old Female. I have been having a brownish discharge after my period. Yes, I am sexually active but have been negative for STD's. When I shampoo my hair, it comes out in the shower. I am starting to freak out. I lost weight kind of faster than I have before, and then it started coming back. My diet's been the same. I don't know what to do. I'm bloated like crazy, but it's usually gone after my period. My periods ended a few weeks ago. What do you think could be wrong? Please help!