Painful bump in back of baby's ear, what is this?

Can't say without ex: Unfortunately an examination would be necessary to answer this.

A lymph node: Lymph nodes are part of our immune system and they get larger when we need to make antibodies to some potential source of infection. Your baby has lymph nodes behind the ears which is normal. However if the area is really red and extremely tender his lymph node may have gotten infected and you need to take him to his pediatrician.