They ain't lie....You are very fertile after having a baby?

I just had a baby 6 weeks ago. I had sex last week, so its been a week. I went to my post p apt. and was told I was pregnant again. Has anyone else had this happen. I really feel like something else is the culprit, we used a condom. No nothing is 100 percent, but dang... Plus I breastfeed my baby, I know i know, that means nothing either....