AE- this sounds abit like a hernia to me, my DS2 has a hernia and has to have an operation on it eventually been told they won't do it until he is almost 2yr old though
with my DS2 inbetween the 2 sides of his belly button is a light pinkish/purpleish colour that looks rounded and stretched like its going to bust sometimes when he has completely over eaten and its all shiny

I will take a pic of his belly button and post it here for you to see if your DD's looks like it so you can have an idea