I had something similar, when that happened to me the doctors told me I had placenta previa. Where my placenta was low lying so I was put onto strict pelvic rest.
Brown discharge during and after sex is typically a sign of your uterus and or cervix being agitated and its shedding little remnants of the lining. Thats it momma :) if you start to experience discomfort and the discharge increases and changes color to a bright red then I would call my doc to let them know :)