Depo Provera or something else ?

Patient: Okay so im sixteen and i got my first depo shot on dec 1 and my normal scheduled period was on dec 8 but i didnt get it , i had sex the day after getting the shot and then every other day after that im not sure whether i was suppose to wait till the shot kicked in or not bcuz i got what seems like my period on jan 6 and its been three weeks with brownish reddish spotting .. is this the shot or something else ..