Thank you for the update, and it is good to know that your vet felt like it was a hernia. I would keep a close eye on it for any injury or worsening. I have had several with those enlarged bellies from one reason or another. We just had another BYCer last week whose hen had one of those swollen...
In a larger flock you may notice a chicken that looks that way. As Wyorp Rock said, the enlarged lower belly can be from a number of possible reasons. It doesn’t always mean that they are in danger of dying right away, but they may have a common condition that many laying hens can suffer...