It looks like a flea allergy to me...I'd get her to a vet for treatment. It's a spot where cats can't clean themselves, so the irritation just continues to grow.

If it's a flea allergy, wouldn't some Advantage, coupled with hydrocortisone or benadryl & something like Neosporin, do the trick?

Have you recently switched cat foods? I have a dog that breaks out from dog food allergies. If no fleas, it could be the food.

Thank you all.
I have ruled out fleas,
Trying a product called, NFZ Puffer. A medicated powder safe for cats.
Clay

just noticed you said you just got back from a 10-day trip;
that might have something to do with it.

Pets sometimes get stressed when their owners are gone, & it can affect them just like stress can give humans pimples.

& if she was boarded where there are other animals, she may have caught something.

I would keep watching & working on it, & if it gets worse, I'd have a vet look at it.