What Causes Spotting, Cramps And Abdominal Pain After Sex?

My girlfriend had her period about a week after we had sex. A few days after we had sex she noticed that she had a brown discharge and later was bleeding lightly and the blood was a light colored red. She has mild cramps and some abdomonal pain. It has been going on for a few days and we are really concerned. She has had a miscarriage before but doesnt know if that could be happening again. Please help us.