Does anyone know much about the depo shot?

Chelcie • 28. Mum to 2 girls 🫶🏼 UK
I had the injection done in May, I was suppose to get it done again August 17th, but my BF and I decided not to, because of the things it was doing to my body. So we agreed to use condoms. 
But last night we had unprotected sex twice and supposedly on my most fertile. We already know we want to have a family together, we've spoken about it all. I think it's a case of if it happens then it happens. 
But my question is how likely is it to get preganant so close after coming off the injection?