It's not that she is more fertile it's that she ovulated before having a postpartum period. This can occur as early as 4 weeks following birth. Ovulation is the release of the egg from her ovary and having sexual intercourse close to her ovulating allowed the sperm to impregnate the egg. How soon after she had the baby did you two have sex? Is she breastfeeding? Some women who aren't breastfeeding will have the postpartum period as early as six weeks following childbirth. But she doesn't have to have a period before ovulation. So in your case, if you had sexual intercourse and ejaculated inside her vagina within 3 to six days of her ovulation she probably got pregnant. Note that 3 to 6 days is how long sperm can stay alive inside a woman's body after being ejaculated.
damn. i had heard that it takes the vagina something like 6 weeks to heal after giving birth. that had to hurt just one week in.