What Causes Vaginal Spotting After Stopping Depo Shots?

I got a depo shot in october and was due for one in januray well the 13 of this month but through out the time i had the shot i had very irregular periods well i had sex last week and no wi had some brown like period and have had cramps and a lil more bleeding nothing to even wear a pad though what is it and why did i spot through the whole time i was on the shot i got off and then had a child then decided to get back on but now i am off again what is wrong?