What's wrong with my poor wee cat?

[RedPandaFluff]

Hi everyone - I've had my poor wee cat to the vet three times about recurring sores on her face; the vet has ruled out a few things but isn't actually able to tell me definitively what's causing them. The vet says that it's likely an allergy, but it could literally be anything. So far, we've ruled out (or at least, we don't think it is):

Flea/mite allergy
Food allergy (tried all sorts - grain-free etc.)
Cleaning products (we've tried different pet-friendly ones)
Air fresheners etc. (we don't use them)
Food and water bowls
She's not been on any medications
I don't think she's stressed . . . but the vet tried metacam in case she's in pain/stressed with any hidden injuries/conditions - but we still had a flare-up.

They seem to suddenly appear every three months or so, look sore and weepy for a couple of weeks, and then heal again.

Does anyone have any idea what else I can try? Vet is stumped and I feel so sorry for my poor little cat

Bump

My friends cat had something solitary but not as bad. They thought it was an allergy to something.

Similar

Thank you, @Toddlerteaplease - we're pretty sure it's an allergy but we just don't know what. It just seems to flare up every few months. Poor wee thing

Does she go out? It might be something she comes in to contact with outside.

She's a bit of a scaredy-cat, @Toddlerteaplease - we allow both cats into the back garden (supervised) but she's quite timid so she'll venture out carefully for a few minutes, and then bolt back inside if the wind so much as ruffles her fur

I don't think she's out long enough to be in contact with anything, and there aren't any toxic plants. Honestly it's such a mystery