What Causes Irregular Periods And Mood Swings While On Depo Shot?

ok i am 20 years old , i have a son and i recentely got on the depo shot. I was told that if i didnt have a period the first few months i wouldnt have one. well i missed my period for 2 months now i am bleeding pretty heavily, having mood swings, my breasts are tender, morning sickness, pain in my lower abdomen, and dizziness. I have taken a home pregnancy test and it came out negative but was also told the heavy bleeding was not normal what could be wrong?