most likely a mite or lice infestation, would get either dust spray for poultry lice/mites, you will need to treat your whole flock along with their coop ,roosts etc, then do it again in 7-10 days for any hatching eggs.
you will be able to see them if you looks closely or their eggs in feathers vent area is usually where they are most heavily infested so check thoroughly.
She could also be having a reproductive issue or starting to molt. mine have just finished theirs including the rooster. The other hens could be pecking her for some reason.