No, I don't think it is very rare. But it might be pretty rare in the first 4 months. BF does tend to suppress ovulation, but it is not reliable. I've heard that women do not get their periods while they are breastfeeding. It is sometimes true.
Some women start up their periods fairly soon after delivery.
The majority of women resume mensturating sometime around the middle of the first year. Breastfeeding is not perfect birth control. Women have conceived before menstruating in their early months of bf. Women have also conceived while using Norplant, birth control pills, condoms and every other method. In most women who mensturate during those first 6 months, the first period isn't preceeded by ovulation, so the odds of pregnancy are very small.