Absolutely ! Frostbite.
Poor baby

The black may still slough off ( peel)
She actually has started to heal !
The white edges around the very reddened area is new ( healing ) tissue.
It will look and act like a burn during the healing process.

You could try to catch her ?
If she were to be treated by a vet it would be with a topical medication called SSD (Silversulfadiazene )

Hopefully with her own immune defenses she can fight this off!

I would think it is painful .
It's hard to tell-but let's hope it involves the superficial skin only.

Reading about human frostbite will help you understand the physiology/ healing process better.
Frostbite is caused by severely reduced blood flow to the skin's surface (the blood vessels constrict to preserve core temperature).

I guess when you got the re-freeze she was was unable to adapt quickly enough?

Best of luck with her, Lynn