Look, you are upset and that is understandable. It's nothing to do with tit-for-tat and more of a realisation that whilst your friend might be your best friend, you are not hers.
I'm sure it isn't deliberate on her part and like someone said, she probably thinks you have enough with the baby although I think that she should have at least told you about it rather than let you find out through someone else.
However you are a part of her big day and you are invited to the hen do. Do go and I'm sure you will meet plenty of her other friends who are similarly miffed that she hasn't asked them to be bridesmaids either. She's probably one of those annoyingly popular people who is surrounded by a posse of best friends.
As for the role of Godmother, well think very carefully about that. I don't know my Godmother or Godfather and I think that's a real shame. I would have liked for them to have stuck around. As a teenager I really needed someone on my side and that's where they could have helped.
I chose the Godparents for my two very carefully. I wanted people who I knew would still be around in 15-20 years time (to the best of my knowledge anyway). People who would take an interest in my children growing up and who would be there for them.
So choose wisely. You don't want to choose a friend who will swan off in 5 years time never to be heard of again. Choose someone who has already supported you through hard times, who has shared their life and all its ups and downs with you. These are the kind of loyal friends (or even family) who deserve the role of Godmother, not the popular girl from school.