It looks like she got her foot caught and hurt it trying to get it free. I would soak it in warm Epsom salt water to clean it up a bit and have her seen by a vet. You want to make sure there are no broken bones if that is what happened. It does not look like any fungus I have ever dealt with and the back foot is typically not a place you would find a herald patch of fungus. It's usually around the anus, mouth, nose, chin, ears.