What Causes Brown Spotting After An Intercourse?

I had sex Jan 7, 2011 and it's withdrwal. got my period by feb and it's normal 3days but very light pink in color. it's already month of may, I still dont have my period for the month of march,april. no signs of pregnancy,negative PT, took it 3x. ive got lower back pain. and now I have brown something when I wipe after urinating. please help me! thank you additional Im sometimes bloated, and I always fart that there's always a gas inside my tummy..