Help with poorly chook please

Hi, have joined today as need the wisdom of more experienced keepers please.
Have three ex-battery for 8 months now, they are fantastic, lay every day, great fun to have around.
Last week i noticed one had began to lose feathers around her face, the skin was not red or sore so just decided to keep a good eye on her.  This week she is eating, drinking, chirpy, laying and seems absolutely fine in herself BUT more feathers have gone, including around her neck.  I had a close look at her and there doesn't seem to be any lice or anything on her, but did notice that around the side of her neck there appears to be what I can only decsribe as an air bubble under the skin, its not a lump as there is nothing at all in the bubble (not water or a mass).  Any ideas?  Am very worried, any advice will be gratefully recieved x

Ok thanks. do you think this is connected to her feather loss?