What does having a long period and bleeding during sex mean?

about 2 months ago i began birth control (depo shot) last month i did not get my period but today is the 18th day of having it. The part i am worried about is that my period will be very light or just brown discharge, but when i have intercourse the bleeding starts up again it soon stops right after the intercourse. I am wondering if this is a sign of something bad or something that is usual with a skipped period the month before. please help i am worried.