What Causes Black Vaginal Discharge?

My last period started may 1, I had unprotected sex on the 14th and the 15th. I had dark brown almost black discharge or blood on may 30th on and off all day, nothing yesterday and light brown discharge today. I did take a hpt about 5 days ago before the noticed dishcarge and it was negative. Is this my period? Also I'm 35 years old.