it's sounding to me like it's possibly Coccidiosis. Did you hatch her yourself? Has she had access to your soil?
Here's some info:
Coccidiosis
Coccidiosis is a parasitic infection in which parasites frequently found in soil or bird feces infect the intestines of domestic birds, growing quickly. It can be fatal but any chicken surviving an infection will be immune to future infections. The
video here may be difficult to watch, but is educational and shows how a symptomatic chicken behaves.
Medicated feed grower feeds medicated with Amprolium do not prevent infection. Chicks receive low doses of medication to better manage infection once chicks are exposed. This gives chicks a better chance of surviving their initial infection and earning their natural immunity. Feed medicated feed until two weeks after chicks are put outside where they may be exposed.
Corid larger dose of Amprolium used to manage infection. It is easier on the chickens system than other treatments. Corid should be your first line of defense if it is available.
Sumlet 12.5% Solution this is Sulfamethazine, which is much harder on the chicken. Only use Sumlet if you cannot get Corid or if Corid proves ineffective.
If you are unable to find either of these locally,
here is a pdf list of medications that may be used to treat Coccidiosis.
Also... it may be a vitamin deficiency/wry neck. If you google this you should find some videos that show what that looks like so you can get an idea if that's what it is or not. You can also search in this forum for "wry neck" and find the treatments.
Good Luck!