My 5 month old babies nipples feel hard. is this normal?

Likely normal: I believe that you're referring to hard little lumps behind the baby's nipples. If this is the case, it is common for infants to have a slight swelling of the mammary tissue behind the nipples. This is caused by response to hormones in the baby's body. Especially for infants who are breast-feeding, the maternal hormones can cause this response. They may also be different sizes; this is common.