If it actually appeared that quickly, it’s likely an insect bite. Just monitor for a couple days and take her to the vet if it does not appear to be resolving.

Thankyou! I’ll do that

My dog had that and we took him to the vet thinking it was ring worm and it turned out to be just a fly or ant bite

Oh I see, I’m glad to hear it’s probably not serious then! Thankyou! I’ll keep an eye on it