It looks like the area under her vent?
How old is she?
Has she started laying yet?
If so when did she lay last?
Is the area warm/hot to touch?
Is she with others?
Edited - Oh it's her crop area. What kind of feeder do you have? Is it possible she's "rubbing" up against it or something? Is she doing the plucking or some other bird?
They do "pluck" when broody but under them; between their legs, the skin against the eggs for humidity/warmth.
Possible but thinking more like rubbing up against the feeder.
Check her vent area & under her wings, push the feathers apart & look close to her skin and on the feather shaft, do you see any tiny crawly bugs? If it is mites and you have other chickens, they may have it too as well as in the coop. But sounds like she's the only one affected ... Have you seen her plucking herself? Have you seen others plucking her? Time to call for some help ....
@Wyorp Rock @WVduckchick @casportpony @azygous