Felt a twinge of jealousy because I found out ex got married and didnt tell me?

Well, it brings up old feelings. He is important to your life because you have a son together... he wasn't just any old boyfriend. And now you feel jealous because he is committing himself another woman when he can't even commit to YOU when you have a son. So, he's not a good guy and you know that, so don't worry... you'll get over it.