The depot shots are hormones ,known to have menstrual irregularity,/lac of menstruation
. The shot is to be repeated every 3 months. so you are hardly out of the priveous shot you had.So possibility of pregnancy
It is also a reversible contraceptive
. The bleeding you have may be starting of your menses.You will resume your regular menses in a couple of months.It seems to be nothing else but withdrawal of the shot.