What kind of bedding are you using? I know it sounds weird to ask, but cedar for example is toxic to chickens. I wonder if her feet came into contact with a toxin of some sort. It could be that she is more sensitive to something than the other chickens and that is why she is the only one reacting. When you showed the bottom of the feet, I didn't see any wounds that would suggest a staph infection from puncturing her foot or anything.
Here's a link to chicken frostbite that doesn't have swelling.
http://www.thecozynest.com/pictures.htm
Here's a link to chicken frostbite that doesn't have swelling.
http://www.thecozynest.com/pictures.htm