It is unusual to get pregnant while nursing, and this pregnancy is a lot closer to her last one than previous pregnancies. Maybe it has to do with her hormone levels changing as she is aging. So, I could understand her being a little surprised.
We love the Duggar family, and, although it isn't the path I would choose, I don't condemn them for it.
I told my husband, when he dropped by for lunch today, that she was preggers again and he said, "That is exactly why they should have a television! They really need to find other things to do!"