What Causes Brownish Vaginal Discharge And Lump In The Stomach?

I have been having a white tissue like discharge and about two wks ago i had a discharge that only happened for one day that was a brownish/yellowish color and had an odor..I am now feeling a lump from time to time in my stomache as if there is a baby there.I am still having a monthly period but cramp all throughout the month.I have taken a pregnancy test and it says negative.What could be the cause of the discharge and the lump in my stomache could i be pregnant but it just not showing up on PT? Could i have had a miscarriage and the fetus still be in my stomache?