I believe when I started the shot the nurse told me to wait 24 hours, but I could be wrong as that was about seven years ago.
Abnormal bleeding is very common when starting or stopping the birth control shot. Some women have spotting for a very long time, others have a lot of blood for weeks, others have no blood at all, and still others have everything in between.
If you are not familiar with the side effects of this drug, you might want to look into that more as the shot can cause irreversible damage.
You can still get a pregnancy test at the clinic to make sure. There's certainly no harm in wanting to be certain.