It looks like feather picking.
Her bottom does look swollen, exactly how does that feel - soft, hard, fluid filled, etc.?
How are her eggs?
Observe your flock interactions, especially at roosting time, this is usually when some feather picking takes place. See if they have enough roosting space.
In cold weather, they tend to hang out in the coop more, being in close quarters can make them pick at one another as well.
You can apply some Blu Kote to her bottom to help hide the redness. Sometimes increasing protein may help, you can give them extra with egg, tuna, mackerel or meat.
Also check her for lice/mites. I agree with @sunflour it appears that there is some new growth coming in.
A cockerel can cause some upset within the flock, so keep watch on him. Bald spots on the head, back and wings can be common. If your hens seem to be upset with his hormone driven behavior, then separate him out if you can. If you don't have separate housing for him, you can always cage him for some of the day to give the girls a little break.
Her bottom does look swollen, exactly how does that feel - soft, hard, fluid filled, etc.?
How are her eggs?
Observe your flock interactions, especially at roosting time, this is usually when some feather picking takes place. See if they have enough roosting space.
In cold weather, they tend to hang out in the coop more, being in close quarters can make them pick at one another as well.
You can apply some Blu Kote to her bottom to help hide the redness. Sometimes increasing protein may help, you can give them extra with egg, tuna, mackerel or meat.
Also check her for lice/mites. I agree with @sunflour it appears that there is some new growth coming in.
A cockerel can cause some upset within the flock, so keep watch on him. Bald spots on the head, back and wings can be common. If your hens seem to be upset with his hormone driven behavior, then separate him out if you can. If you don't have separate housing for him, you can always cage him for some of the day to give the girls a little break.
