What Causes Brown Vaginal Discharge While On Depo Shot?

I had my depo shot in april 21, and i have been having brown discharge for about 2 weeks and I am now starting to bleed bright red blood? I have been on the depo since October? I took 2 pregnancy test and both were negative. Had a pap smear april 20, and everything came back good on that! What could it be from?