plan b pill

So I had my baby 6 weeks ago (ftm) my doc asked me if I wanted birth control & I said no because of weight gain & I'm already a little over weight and I'm trying to get that number down as of right now. So long story short, me and my love had sex with a condom but it broke so I took the plan b pill. Has this happened to anyone? How affective is the pill right after sex?