A grey squirrel got into my coop. My chickens went after it, later that evening my husband noticed my Buff having a hard time walking. I believe she probably got bit. She is not seeming to be doing very well today.
At the risk of asking a dumb question, have you looked her leg over for puncture wounds? Bacteria entering a flesh wound can kill a chicken in as little as 24 hours.
I would check over the chicken from head to toe and see if you can find any puncture wounds. If you do find them you will need to treat them instantly because bite wounds from any type of animal are the most serious due to all the bacteria that is in an animal's mouth. If possible I would bring her to the vet, she might need antibiotics.
Ditto, give that bird some antibiotics. And try to get rid of those squirrels. Squirrels have been known to steal eggs and even chicks, especially when they have babies and need the protein.