yes you most certainly can. Although chances are lessened during this time, breastfeeding is NOT a replacement for birth control. Just because you're not having a regular period, this does not mean you will not randomly ovulate and become pregnant. If you are breastfeeding make sure to be using some method of birth control if you do not wish to become pregnant again.
I should add I got pregnant 3 months after my first son was born, and I was breastfeeding. I also wasn't even menstruating; so I have no idea how it happened. My daughter is now 12. =)
To throw in my two cents, I've heard that factors like co-sleeping, breastfeeding on demand and whether or not the child is eating other food yet can impact the likelihood of getting pregnant while breastfeeding (the first two reducing the chance and the third increasing the chance if the baby is eating other foods). That's all just hearsay but I found a couple pieces mentioning breastfeeding exclusively as being a factor:
http://www.mothering.com/breastfeeding/eco-mama (about 3/4's of the way through the article) http://www.who.int/features/factfiles/breastfeeding/en/index.html (click on "read more about breastfeeding link" - it's in fact #3)