What’s going on?

Hey ladies I had a few questions... I am sexually active and the last time I had sex was July 13th, I was on my period and my period ended July 16th. Then I had an extremely light/spotting “period” the 28th through the 1st. Now I’m having brown/red discharge on and off. I took a pregnancy test a few days ago and it was negative. Why could be going on?