Hi
I've had a similar experience, with a chicken just standing puffed up, head under the wing, chickens of different ages, and one who was lying down a lot, moving places but not doing much else, breathing heavily. It has happened maybe three times in all the time I've kept chickens, and I'm sorry to say in all three cases they didn't make it.
When they are so ill they aren't behaving normally I think the illness is already too far gone. I had one young chicken out of a clutch of 10 I got very recently that seemed to be the loner, sitting quietly a little hunched up, but I kept a close eye, watched for bullying cos chickens are sometimes just plain mean to each other.
a lot of the time it was joining in with the others and eventually I couldn't tell which one I was worried about. But then out of the blue, it didn't come out of the house one morning and it never got better.
All the others are fine, I wondered if it was the really horrible wet weather we were having at the time, but I made sure they had plenty of food, corn and even warm porridge with live yoghurt to keep the chill off and help their immune system.
As long as it's an isolated incident I put it down to nature and survival of the fittest, I do keep quite a few chickens and the odds of one becoming ill are more than when I just had a few, but I always wonder if there was more I should have done, noticed earlier...
I can't get a vet to take the idea of a PM seriously so I just have to make my own assumptions and try to give the chooks the best care I can all the time.
I try alternative/complementary products as opposed to reaching for antibiotics every time I see a problem, and for prevention, more often than not they work - Nutri drops, colloidal silver, citrus drops, garlic, cider vinegar, they are all in my chicken medicine cupboard - try your local feed/farm store place or 'happy chicks' for mail order
Good luck with yours
Helen