Hen having trouble moving right leg

[Jet the Austrolorp]

We have a lovely, sweet Austrolorp, just about 3 years old, who started limping six weeks ago. I thought she'd broken her leg or dislocated it. Then she got very sick, breathing hard and unable to eat much. I took her to a vet who x-rayed her and found no problems with her skeleton. Another vet read the x-ray and said it looked as if she had fluid in her abdomen. That probably accounted for her breathing difficulty. That seems to have gotten better and she's now eating, but has very little energy and drags her right leg all the time. Sometimes she stands on the foot bent backwards because she can't swing it all the way forward. Other times she can't swing the leg forward at all, as if it's stuck and she can't bring it forward. It doesn't seem to be getting worse, but it's not getting better, either. She's been in this state for about a month. Any idea what this might be? She's a really sweet, calm chicken. She has never once picked on any of her flock and we feel so bad for her.

 

[Lacy Duckwing]

Is your hen vaccinated against Marek's disease, and if so, when was she vaccinated? Have you introduced any new birds to your flock recently? Is she showing any other signs of illness?

With her abdomen, does it feel hard or bloated? When she pecks the ground, does she keep her chest up, or does she allow it to lean toward the ground with the rest of her body? Do you know if she is still laying eggs? Is she eating and drinking just fine?

 

[Jet the Austrolorp]

Thank you for all the specific questions--answers to which I should have included in the first post!
She is not vaccinated that I know of but I doubt it could be Marek's since none of her four flockmates has shown any symptoms and I understand Marek's to be highly contagious. Plus, these five never come in contact with any other birds. The only other sign of illness is her lack of energy but she was quite sick a few weeks ago--heavy breathing, loss of appetite. That's when I had her x-rayed and the vet said she might have had fluid in her abdomen. That hard breathing has gone away. We think that one week ago she layed an egg (hers are a bit different in shape than the others) but nothing since then. She is eating and drinking, but hard to tell how much. She gobbles down her favorite kale which makes me hopeful. When she pecks the ground or the grass, she does go all the way down normally. Her abdomen does feel hard, but she is very, very thin. The vet told us that and suspected that she might have been sick for quite a while (that was back when she was breathing hard).
Again, thanks so much for asking all your questions. I do so appreciate your concern!

 

[Lacy Duckwing]

Tagging @Eggcessive, as she's more familiar with the internal stuff of chickens.

 

[Eggcessive]

Sorry about your lame hen. It is more unlikely to be Mareks in a 3 year old unless you have added a bird in recent months to the flock who might have been a carrier. It could be an injury, a tendon issue, or something (a tumor or infection) could be pressing against the sciatic nerve in the leg. Hard to know. But if you lose her, it might be helpful to get a necropsy to find out what might have been going on. Some do their own at home, and then post pictures here. Here is a list of state vet labs where a necropsy may be done:
https://www.metzerfarms.com/poultry-labs.html