M.M.
It is normal. Some women just have a stronger cycle than others. I have a friend who has two children and got her period 4 months after each child and she breastfed both children. I myself breastfed my son and got my period when he was 4 months old and breastfed until he was 7 months old.
Every woman and her body is different.
Try to google "web MD + breastfeeding + period", there is some good factual information there.
www.breastfeed-essentials.com/menstruation.html
www.medhelp.org/forums/Maternal/messages/35899.html