Depo injection

hi, basically last month me and my partner did have intercourse but he cum before he entered i no something could still have happened but not as likely. Not thinking there is a chance but 4 days before my period I had my first Depo Injection  then started to get brown spotting but then it stopped. I have had no period yet I have been on the Depo before and it always had regular periods. So I’m wondering why this time is different? Has anyone experienced this? Could I have been pregnant before the depo was done but was too early to no?