What Causes White Vaginal Discharge And Stomach Pain?

Hello I m 19 years old and I have been on the depo shot for awhile but I stopped taking taking it back in april?? I thought I was pregnant til I was spotting which only happend 3 or 4 times and my stomach hurt like no other I have had a brownish/pink and white discharge with pains like cramps but they arnt have takin a pregnancy test and it said negative but it could be to soon.. any ideas of what it could be??