It looks like her left leg is lame, maybe even paralyzed. Can you tell if she can move her toes if you stroke them? She is also standing on the hock or elbow when she stops to eat. I would be suspicious of a badly hurt leg, or possible Mareks disease. Do you know if she was vaccinated for Mareks? Have you added any new birds to your flock in the last few weeks? There are a couple of larger roosters in your flock, so an injury could be the problem. You may want to pen her up for a week or two with food and water where she can still see the other chickens, but she can rest her leg to see if it gets better.
