Inspect her closely for mites and lice.
It could be that external parasites are itchy and she's scratching the area with her toes.
Check the vent for live ones and inspect the feather shafts for clumps of eggs. Look under her wings close to the body.
Tell us what you find.
If you think its mites start treatment of your whole flock, I treat everything of mine every 3 months with ivermectin. Its a cattle pour on and goes on the skin and is absorbed through the bloodstream. I use it on all poultry. Results 3-4 days. I had a baby turkey and no matter what I used the mites on its head wouldnt die so ifter
Sorry posted too early lol
So after using ivermectin the mites were completely gone. I think it also helps with intestinal worms also? I had noticed before using ivermectin I would get the occasional worm in poo and now I dont see any.
Ivermectin is used for worms/mites/etc with dogs. Be careful with the dose, application, etc. as for dogs it will kill some breeds or make them very sick. Not sure if the same holds for chicks.
You could try to separate her out & give her a bath with environmentally friendly/dawn dishsoap. Use running water to help flush the mites off of her as best you can. I read somewhere about using a product called nustock I think it was. I've had mites on my sex-link that I've had luck treating with batch & coating (working down to her skin) with petroleum jelly then washing & repeating in a few days. The petroleum jelly can/will dry & flake off, but she might not like that. Just treat her all over as they'll move to an untreated area. If nothing else, might help give her some relief until you can get her to the vet after the weekend.
If you google at home treatments there are loads of suggestions.