Pregnant while on birth control?

Hello my name is liz and i'm 19 years old first of all let me start by saying thank you for taking the time to read this......anywho I've been on the DEPO shot (birth control) for about 5 months during the first 3 months i didn't have a usual PERIOD but i at least always spotted once or twice a month for about a day or two...In the last 2 months i havent spotted at ALL and i have alot of cramps, I urinate more than usaul ive been sort of nauseaus and I recentely noticed a milky color substance discharging from my right BOOB....In the last two months my partner and i have always taken certain measures so no mistakes are taken place for example we use condoms or he pulls out before he ejaculates...I also took a pregnancy test about 3 days ago and it came out negative is there a possibility that i might be pregnant or is my body pretending to be pregnant??