Started birth control

Harmoni
Ok well I started birth control (the shot) over a month ago and well I was informed you aren't supposed to have a period(my reason for the shot). That was until I started having dark brown and black blood come out. It was two weeks late than my normal period. I used to have around 5 day periods. It has been 8 days and I'm still bleeding. It isn't constantly but it hasn't shown signs of slowing down.