Unfortunately it sounds like Marek's disease. She's certainly the right age for it.
I have two in a similar state at the moment, a young pullet and a cockerel. It's caused by a herpes type virus which can have a number of different symptoms, but common ones are inability to control a leg or a wing, wry neck and/or wry tail and wasting despite eating well. There is no recognised treatment although people have reported some improvement with vitamins and/or herbal/homeopathic remedies. I've tried turmeric and black pepper in the past to try to prevent tumours developing but they get sick of the taste after a while and stop eating so now I just give them supportive care, as much access to grass, sunshine and good food as I can arrange and they do best with the company of other chickens. It is however extremely contagious and easily transmitted, so you have to make a decision as to whether the rest of the flock have already been exposed. I've had it in my flock for 2 years now, so I don't isolate sick birds unless they are being picked on and then I usually put them in a pen on the lawn where they can still see the other chickens and move around but feel safe and have unlimited access to food and water. Scattering food in and around the pen so that the flock comes and eats around them, helps to encourage them to eat and also move around rather than just sit or lie next to a feeding station. I have had the best recovery results with this set up but you need to be prepared for the worst because it is often fatal. I had to euthanize a young pullet a fortnight ago because her quality of life was no longer sustainable. On a positive note, I've also had some make quite miraculous recoveries pretty much overnight and others that took months of TLC but were eventually able to free range with the flock again and laid eggs. Their lifespan is usually limited by the disease though and secondary attacks are usually more serious and prolonged and are usually triggered by stress.
There are several really informative threads here on the forum about it. If you put "Marek's Disease" into the search field, you will find them.
Good luck with your chick.
Barbara