Thank you for the question and sorry for the trouble. I am DrD , will try to answer to your question. Although I can not diagnose your bird problem here I have some ideas what might be going on . I would like you to check this out. https://extension.unh.edu/resources/files/Resource000791_Rep813.pdf
All symptoms you have mentioned fits into a disease called Marek's disease which is a very common in backyard poultry. Your bird sounds like having the neurological form .Signs are slowly progressive like in your chicken. This is quite a common backyard poultry disease, overall. Typically, signs are slowly progressive, and often involve the ischiatic nerve(s) initially.
There are other things to rule out though . Your vet probably will fecal direct exam and flotation performed; run a CBC and good biochemistry profile to seek any hint of other disease processes that may be in play. Make sure the bird truly is eating / drinking well, and offer a good nutritional plane of diet for her in the interim. Please do not give any other medications since this may worsens her condition .
The virus is primarily spread by infected dander (dead skin cells) from other birds. However, it can be transported in dirty hen carriers, on clothing and boots, wild birds, and by darkling beetles in the henhouse.
It can survive for up to 65 weeks in ambient temperature in coops and for years in soil.
The virus is spread horizontally- that is chicken to chicken, not from the hen to the egg. So the virus will spread quicker among hens in tight living conditions- make sure to seperate the sick one from others. But if this is Marek's than the disease is already there. Soem birds are resistant to the disease and may not show any signs.
I hope this helps. I am hoping that this is not a Marek's case :(