Ay, napakabilis ng timeline na yan, pero honestly kung on injectable ka talaga and consistent sa schedule mo, ang chance ay very low, mga injection ay pretty reliable pag taken on time. That said, the only way to know for sure is to take a pregnancy test once you've missed your period, or better yet, ask your OB directly kasi sila ang makakabigie ng peace of mind mo based sa actual timing and your injection schedule. A few of my mum friends who are on injectables din naman, wala silang worry-worry dahil alam nila ang protection nila, pero of course walang 100% ang any contraceptive kaya if you're really anxious, might as well see your doctor na para malinis ang loob mo!