What Causes Black Color Vaginal Bleeding After Menstruation?

Okay I had sex with this guy Aug. 18 then I was worried but I didn t take nothing . I took a pregnancy test and it came out negative then the 25th I got my period it lasted a week. When I finished like three days later black blood was coming out and I dditn pay much attention and right now it s coming out very red . I m worried . Help me please