Apple cider vingar in the water is good ( do it for a week) also you could chop fine a onion and give her some natures antibiotic. This can be given to all your hens once a week.
My hens have the trots if they eat lettuce but they do love it so its rashoned. Have a think what extras you have given them?
If her tail feathers are up its not to serious if they are down shes not too well at all. May be a chill with all this weather.
Watch out for dirty bum, some hens dont like cleaning themselves so you may need to wash and blow dry it.
change all bedding and nesting material I do mine weekly in this weather it does get damp and not nice for them.
lastly are your hens due for worming?
Hens can get stressed out if moved out from the others I only do it as a last resort.