Pregnancy possibility? 😬

Last night I got a little carried away with my boyfriend.. He just came back from airborne school, it had been 5 months since I last spent an actual day with him & I was planning on just going out for a small romantic date. Weeeell, one thing led to another and next thing you know, we’re having sex & he finished inside of me. 🤦🏻‍♀️ I’ve been on the depo shot for about 3 weeks- almost 4 now. Obviously there’s still a small chance of pregnancy, but is it a big one enough to take a plan B or something? Although I do want children, trying to finish my education degree and be a mother at the same time would be very stressful & it’s not something that’s crossed my mind yet.. a little insight ?