What Causes Amenorrhea After Stopping Depo Shots?

Hi after my baby was born i went on the 3month injection,it didnt work for me because there were spoting on my underwear(everyday).after nine months i decided to not take the depo anymore...its been almost 6mnths since that i havent had my period or any sign of spoting,me and my bf doent use condoms nor does he pull out.please i need advice