Depot shot is very much effective in preventing unwanted pregnancy
and to be taken once in 3 months. It has failure rate less than 0.3 per HWY. Though it takes some times for fully effective, it is advisable to take some back up method like condoms etc for initial 1-2 months.
Some amount cum or ejaculation can make you pregnant if it occurs around fertile days or in case of failure of depot shot. You need to wait for next period to be sure.
If your next period delays more than 7 days from expected date, then you need to undergo home pregnancy test or blood test
to rule out pregnancy. Following test report, you can consult with your gynecologist.