Sorry to hear about your chick, and sorry you haven't received a reply sooner. How is she doing today? Could you answer a few questions that will ultimately help us, help her in the long run?
What is she being fed? What kind of bedding substrate is she being kept on? Do you have a picture of her pen area? Have you inspected her legs, and felt to make sure each joint bends properly? Do you feel any warm spots or signs of redness? Is she eating and drinking? How does her feces look, could you post a picture? Could she have injured herself on something?
With chicks, there are several leg problems that we tend to see often, which would include riboflavin deficiencies, Mareks disease(very unlikley here), slipped tendons, or injuries. Basing off of the timeline in which she developed these problems I would be inclined to think it would be on the injury-related sign, but currently, that's pure speculation, so I think we should cover a few base's and start her vitamins and start her on supportive care.
As just a very basic treatment, I would start her on vitamin B complex, vitamin E, and a natural selenium source to see if it might be nutritionally related, and if she is totally unable to stand up making a sling of sorts (picture provided below) may prove beneficial to prevent sores from developing on her underside. For a vitamin B dose, you can get some Cattle B complex and give her a drop or two alongside her beak, or crush a 1/4 of a B complex tablet and add it to some treats, or mix it in with water and give her a few drops orally. For vitamin E, you can get some 400iu soft gel tablets, and give her one a day over treats or along the break. Offering mashed scrambled eggs will provide her with a selenium source along with several other essential vitamins.
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