I don’t see anything obviously wrong with her feet, but feel up and down her legs for anything swollen or out of place. Feel her keel bone, ribs and hips. Compare to another hen. She may have had an accident or fallen.
It’s hard to tell, nothing is seeming obvious about her immediately, but she could be panting bc she is in pain. Panting can also be seen in egg binding, as well as leg weakness, (but this is less common), so work on getting some calcium into her.
Also work on getting her to drink some water with electrolytes and a little bit of sugar mixed into it. Keep her somewhere away from the others for now so she doesn’t get picked on.
Get a picture of her poop when you can.
Edit: also, is there anything toxic she could’ve gotten into??