:hugs
Sorry for your loss - it is always difficult. Chickens never tell you when they are sick :(
With what you describe I'm leaning towards a reproductive problem like internal laying - about which there is nothing to be done sadly.
A little yoghurt can be good for the probiotics, but chickens...
If she passes (hopefully not) you could get an autopsy done by your local State Extension service to determine cause of death....
When you clean out the coop make sure it is well aired out before you put her back in, sometimes the fumes can be irritating to the breathing system, :fl
Are they pooping? If so what color/consistency etc. Is it particularly hot in your area right now?
Have they been laying regularly up to now?
Try making a wet mash (oatmeal consistency), add some electrolytes or poultry drench and feed it to them slowly.