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 bellies, that had gotten injured and ripped open, exposing her organs. Unfortunately she had to euthanize her hen, since it was not treatable:
https://www.backyardchickens.com/threads/please-help-insides-coming-out.1272375/
https://www.backyardchickens.com/threads/please-help-insides-coming-out.1272375/