The little chick is eating alot.. just still bouncing around. how many days would you give her?? in the er pen.
I would make sure she can get away from the other chicks.
Like Capayvalleychick said, she could have had Mereks. The Vaccine does not stop them from catching Mareks, it stops some of the Cancers. Nearly all chickens get it(especially if vaccinated)--most don't show symptoms. The Cancers start at 9 months of age or so. Already this year there have been several cases of chicken cancer that was probably Mareks related. The picture of the chick with the leg straight out and not being able to walk is a symptom of Mareks infection.
Ron