I can't help regarding diagnosing a yeast infection in dogs, as I haven't experienced that, sorry.
However, I have a 22 week old girl, who has had skin problems since we brought her home at 8 weeks. ( clusters of bumps on her skin and dry scaly patches and hair loss from scratching ). She was diagnosed as having a staph infection with two separate antibiotics, neither of which cleared it up. We were told by the vet that she was too young for food allergies, but I was not convinced, and through trial and error we isolated that she has an immediate and definite reaction to eggs for one. We put her on a grain free, egg-free diet and she has been doing really well the past couple of months, with great skin and coat.
Anyway, to get to the point .... This week she developed a really angry rash in her groin and under her arm-pits after I fed her chicken for several days in a row. She has eaten chicken many times before, but sporadically, and never every day. The rash started out as clusters of pimple-like red bumps, and they have now turned crusty. She itches like mad. She saw the vet last week and after excluding other possibilities, they have concluded that it must be an allergic reaction and acknowledge that despite her age, she probably has food and environmental allergies. They have taken blood for allergen screening and we are now just waiting to see what the triggers are.
An allergy can suddenly develop with a food that has been previously tolerated well ... in fact, sometimes, the food needs to be eaten for a while before the sensitivity is triggered. So, just a thought for you to consider, and maybe worthwhile getting allergy screening done .... if it should prove to not be a staph infection, or yeast as the culprit.
Jinny