@LozzyR oh, how heartbreaking! I'm so sorry to hear about BessieDear all, I’m gutted to say that Bessie died at the vet’s today. She lost interest in eating and drinking yesterday so I gave her some pain meds at bedtime. She wasn’t interested in food and water this morning so I gave her another dose of meds. She seemed to pick up a bit and had some oats and yoghurt. I had to go out today and when I came home I put her in the sunshine next to some juicy grass. She didn’t show the slightest interest in it and her breathing looked a little laboured. Our regular vet is off today so I took her to see another one who has some experience with chickens. The avian vet thought she had cellulitis and recommended a stronger antibiotic. The attending vet suggested this course of action and went to clip her vent feathers and get some fluids into her. He came back and said she couldn’t breathe on her side and thought she may have a lung infection as well. He thought she had made the decision for us and brought her to me wrapped in a towel so I could say goodbye. She didn’t regain consciousness and died in my arms.
Goodbye my little Bessie-Boo, you gave me so much joy in the five months you were part of our family. I’m sorry I couldn’t do more for you. xoxox

You did what you could for her seeking vet care, medications and giving her lots of love. Sometimes in spite of our efforts they just don't make it.
I too would see about getting a necropsy to see if you can get more information.
She was a lovely girl, I have a soft spot for Barred Rocks.