Depo shot

I'm on the depo shot. After the three months are up I am just going on the pill. I was reading the directions and it said to get it either 1 or 2 weeks after giving birth. I got it when my son was around 2 months. I had a period before hand but now no periods. We were using condoms for 2 weeks (the nurse said to use them at least 2 weeks) and last night we didn't.  but I was reading that someone got pregnant on the depo. I'm scared about becoming pregnant. How many others have gotten pregnant? And even though the directions say that am I still safe with it? Thak you!