What Causes Mood Swings,nausea,indigestion,cramping And Light Bleeding?

hi i stopped taking the depo shot i was suppose to get it july 23 but didnt and i have been feeling depressed and mood swings and nausea and indigestion and i started bleeding it started light pink then red then brown then red again and it isnt heavy it is light and i have been cramping before i started and had more head ache then usually and have had lower back pain and they are very painful sometime but usually it hurts but not to bad and i was just wondering if its from the depo shot or if i am pregnant because my step mom got pregnant after a week of being off it