Hi Skelley,
Do you suspect the feather loss is from being picked on or from something else? Does she have peck wounds- small red dots or bleeding feather stems? Could she just be molting?
She could just be molting- I have some of my birds molting in patches on their bodies. If it is just a molt, she could just be feeling under the weather because of it. My hens, when molting, don't seem like their usual selves. Make sure a molting hen has some extra protein- something that should be supplemented in the winter for birds too since there aren't bugs out and the ground is too frozen to dig for worms.
A good 'test' to see if a hen is ill is to give a treat- yogurt, oats, raisins, meal worm... etc. If the hen goes for it right away, then she is probably not sick. If she takes her long time getting to eat it, or doesn't go to eat it at all, then I'd say she is probably battling an infection or is ill somehow. My healthy hens usually don't act lethargic when I bring them in the house, though I do think they are kind of conscience of being somewhere new and not knowing what's what and won't really be as comfortable as they are when they are in familiar grounds.
Bringing her in is a good thing to do, especially if she is being picked on constantly. I had one hen who was very friendly but as she grew, she was picked on very badly from the other hens. I had to separate her from the others. Sometimes hens fight because of ranking order, other times they 'smell' or sense illness in that hen and try and push her away from the group. So keep that in mind.
Hope someone else can chime in to help.
You are doing the right thing by looking after your hen. I hope that she has nothing serious and will feel better soon... something that I've had happen to me with my hens multiple times... sometimes they are just not feeling well like we all do from time to time.