I would immediately start your bird on liquid lysine and b vitamins. I just had this exact situation- thinking my bird had hurt a tendon. It went on for two weeks and then the other leg went. It was Mareks in my case. My bird has been in a sling (google sling for mareks) for over a month now. She is eating and doing well but I really wish I had started treatment sooner. If you are up for the challenge of trying to keep a Mareks bird alive it is really well worth it. Scientists are not able to study Mareks the way we back yard flock keepers can. However their have been numerous studies on Mareks and Lysine by big agricultural companies and they are the reason commercial bird food now contains lysine- to help combat mareks paralysis in giant industrial farming concerns. Remedies for actually keeping an individual bird alive are not being studied except by individuals. I am working with a poultry professor from my local vet college and she seems to think that by the time a bird has actual symptoms of Mareks, your whole flock has already been exposed. If they are vaccinated there shouldn't be a problem but culling the bird now wont keep the other birds safe. My birds were vaccinated. I was told by the professor that the vaccine probably failed for this bird but is working on the rest of my flock. By trying to keep her alive we can learn a lot about the disease. As long as you don't think your bird is suffering and should be euthanized for that reason I urge you to start treating her as if its Mareks. If its not, none of the treatments will hurt her.
I give my Mareks girl:
Super Lysine + by Quantum Health, liquid lysine, 1/2 a dropper full twice a day. Mareks is a herpes based disease hence lysine.
liquid st. Johns wort, about an 1/8th a dropper full, by Natures Answer. This is based soley on anectdotal evidence via the internet that it has helped. I have talked to numerous herbalists and none think it will have any effect on the virus but who knows. If it doesn't counter Mareks at least it keeps her from getting depressed!
Liquid B complex by Now. Riboflavin seems to be the most important vitamin associated with Mareks however all the b's help keep her healthy.
Dog safe buffered aspirin from Petco. Each pill has 60 mg of aspirin. I googled aspirin for chickens and found others using it. If she has been having a lot of spasms I give her aspirin which really calms her right down and seems to soother her legs and make them more pliant.
I am also giving her CBD oil by Care By Design with an 18 to 1 CBD to THC. I am in California so I can obtain it easily. Not sure if it is available in other places. It is a cannibus extract but can't get anyone or thing high. It is associated with helping with epileptic fits and is safe for children so I figured I would give it a try. My girl has spasmodic paralysis, meaning her legs suddenly shoot out above her head. CBD oil seems to lesson the frequency of these fits. I may be the first person trying CBD oil on a Mareks chicken.
I really urge yo to start your chicken on lysine and b vitamins immediately. Then if it is just a muscle issue no harm done.
I feed my girl bread soaked in yogurt which seems to help her digestion. She had loose stools and that has really helped. Some sort of probiotics is a good general thing to do during sickness to keep digestion healthy. Good luck!