Are you wanting to have another baby right away? If not, I would use protection. As the first poster said, you are extremely fertile after having a baby.
If you are having unprotected sex there is always a possibility of getting pregnant. If you don't want to get pregnant again immediately (which would be dangerous for you and for the baby) you should use protection.
Yes. You are most fertile right after delivery. Unless you want your babies 9 months apart, be careful!!