My chickens underside has lost feathers and looks irritated

About a week ago I noticed my plymouth barred rock girl had lost some feathers and was looking a bit red on her underside. I figured maybe she scratched it or something and it would go away, but it hasn't yet and I'm getting worried. The little patch of skin looks pretty bad, very red. None of my other chickens have it so I can't figure out what it is. I'm wondering if theres maybe some cream or lotion or neosporin I could put on? I took a picture, so I hope that can help give you a better idea. Shes laying eggs and very perky, so it doesn't seem to bother her.


EDIT:
1) What type of bird , age and weight.
Plymouth Barred Rock, 2 years, not sure, but average?
2) What is the behavior, exactly.
Losing feathers and redness on underside
3) How long has the bird been exhibiting symptoms?
At least a week
4) Are other birds exhibiting the same symptoms?
Nope
5) Is there any bleeding, injury, broken bones or other sign of trauma.
No just redness
6) What happened, if anything that you know of, that may have caused the situation.
No idea
7) What has the bird been eating and drinking, if at all.
Shes eating just fine, free ranging and layer pellets and drinking
8) How does the poop look? Normal? Bloody? Runny? etc.
Normal
9) What has been the treatment you have administered so far?
I just gave her a mite dusting, it seems to have reduced the redness, I'm thinking she may just have some poultry lice
10 ) What is your intent as far as treatment? For example, do you want to treat completely yourself, or do you need help in stabilizing the bird til you can get to a vet?
Treat her myself
11) If you have a picture of the wound or condition, please post it. It may help.

12) Describe the housing/bedding in use
Wooden coop with roosts and hay and aspen wood shavings for the nesting boxes


And this girls top of the pecking order, nobody messes with her