What Causes Brown Vaginal Discharge?

I've never really had normal periods as i was in the injection for a year, then had one depo shot. ive been off the depo shot for a month now. 6days ago i had unprotetected sex. since yesterday i've had browny/black disharge. i'm going to take a home pregnacy test next week. but do you think there is a chnace of me being pregnant.