feeding, period hasn't returned- could I be pregnant?

After I delivered my son my periods never returned. After my son was 1 1/2 years old and still nursing, I quit "being careful" not to conceive and got pregnant right away. Despite not having a period, I got morning sickness and all the usual signs - tested and sure enough was pregnant. Its totally possible. A period isn't really required to ovulate. I asked my doc about it and he just said "it happens". My sister got pregnant the very first time they had post partum, she was feeding too and thought she was safe!