Poor dog! -- has a vet seen this? - It looks quite nasty.
If yer GP vet doesn't have an excellent
idea of the root-cause, i'd ask for a referral to a k9 dermatologist - if U have vet-insurance, this will be covered.
If it ISN'T too horrible - I-O-W, she's not in pain, she eats, drinks, can sleep normally, etc - I'd even ask my GP-vet for symptomatic relief, then i'd BUY A POLICY that covers dermatology consults & treatment [Westies & SharPei are typically excluded], & wait out the introductory period.
Then, i'd ask for that dermatologist referral, & go get a proper in-depth work-up, Dx, & Tx.
If yer own vet diagnoses it before the policy kicks in, it's a "pre-existing condition" & cannot be covered.
But if she's mizrable, she can't wait for that, she needs help now; U know Ur dog, how is she feeling?
I sure hope it's something simple - but it looks ugly & complex.