Bumblefoot or Leg Injury?

[Birds of a Feather]

There are many different possible causes of the symptoms you describe. Do you have a picture of your pullet?

 

[rebrascora]

You might want to research Marek's disease. It is one of the commonest causes of lameness in young birds and her panting would also point to that diagnosis assuming she is not just overheating. Marek's suppresses the immune system leaving birds prone to respiratory disease as well as coccidiosis etc but the final stages of the disease usually involve the development of tumours and the lungs are one of the more usual sites for them. Keeping her as stress free as possible and well nourished with a good vitamin supplement is pretty much the best you can do.

It may not be Marek's but it is a disease that people should be aware of because it is incredibly common and widespread and whilst it has a huge range or symptoms, lameness or an inability to control a limb is one of the most classic signs. It is often referred to as paralysis, so people think the limb will be stiff but that is not always the case. Sometimes they are just unable to bear weight or act drunk. It is not always a leg that is affected either, it can be a twisted neck or tail or dropped wing or even something as subtle as not being able to keep one eyelid fully open or focus one eye.

The disease mostly affects juvenile birds but adults can show symptoms later in life and be carriers and the disease can have quite long dormant phases in between becoming infected and showing symptoms (having an outbreak) or between outbreaks if they are lucky enough to survive their first outbreak. These are usually triggered by stress which is often due to integration into an adult flock or surge of hormones at point of lay or the unwanted attentions of an amorous cockerel etc.

I hope I am wrong but it is best to be well informed about this disease because if you keep chickens and raise chicks it will likely rear it's ugly head at some point unless you are lucky.

 

[Birds of a Feather]

I agree that Marek's disease is a possibility to consider. This article has some more information about Marek's disease: https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/

It can manifest as lameness or paralysis in one or both of the chicken's legs without an obvious injury. Of course, there are other possible causes for these symptoms, and it is not really possible to diagnose what's wrong from just a description. It could also be just an injury or another non-Marek's-related issue.
Supportive care and poultry vitamins are probably a good idea, even if it is Marek's.