Maybe she relied too much into the transient amenorrhea and protection offered by high serum levels of prolactin during the first few months of exclusive breast feeding.
It happens with a lot of women, they end up relying way too much in that protection offered. A person I know has just gotten pregnant for the second time because of that.
I wonder why people are so judgemental over her though.....it's her life and she decides whether she wants kids or not. And one isolated moment of carelessness doesn't necessarily make her a bad mother by default in my opinion.