It looks like a trichothecene infection, also known as fusariotoxicosis. Various species of fusarium along with several other molds produce mycotoxins that differ in severity of toxicity. This is a common condition in chickens, the mold is found in a wide variety of seeds and grains, it is also found in the soil. Contaminated chicken feed is the most common source of fusarium.
The first sign of fusariotoxicosis is irritated skin around the mouth as seen on your hen. Chickens will refuse to continue eating trichothecene contaminated grain as it results in irritations. Offer good quality, uncontaminated feed and the irritation should heal itself. If the toxins persist she may get watery eyes and ragged wing feathers. Stretch out her wings - are the feathers neat and tidy or ragged? If they are neat the condition is not severe.
As suggested before, it could be canker. This is some of teh symptoms for Canker: Loss of appetite, rapid weight loss, weakness, darkened head, extended neck, frequent swallowing, watery eyes, foul smelling discharge from mouth, white or yellow sores in mouth and inability to close mouth.
