Chance of pregnancy after birth, using withdrawal method??

I just had my son 3 1/2 weeks ago. I understand you're suppose to wait the full six weeks but we didn't. We've been using condoms to prevent infection and obviously pregnancy. The other night we had a condom break ("her pleasure" more like her pregnancy scare -.-) but I felt it pop so we took it off and put on a new one before finishing. Tonight we got into it really quickly and ended up not putting on a condom. (Stupid us. I know. No need to lecture me) He pulled out. We use to use the withdrawal method and I never got pregnant until we started trying but I know you never know when you're gunna ovulate after birth. My postpartum bleeding also just stopped last night. What's the chance of getting pregnant from this?