Frostbite? Spreading fast...

[WallacetheKhakiCampbell]

It may well have been a touch of frostbite but as @Wyorp Rock said:

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The circled part above looks like it's been scraped or banged into something sharp. I would look around your coop/run real good to be sure there aren't any obvious places where she (or any others) could get injured again. Sometimes an encounter with hardware cloth can do this, too.

As long as it's healing ok though there's really not much else to do.

Thank you! I will check thoroughly. She is looking better every day now. I didn't end up separating her but a weird scab like thing ended up covering almost her entire left side of her head. But now its just a bit of a scab over her eye which is still shut closed. I think she will be fully healthy and clear of it within the week!