Yeast (candida) is a normal part of the digestive Flora and, in a healty dog or person, is kept 'under control' by a strong proliferation of the beneficial bacteria. Your dog will have a yeast problem when the necessary bacteria is destroyed by outside factors, most frequently vaccination reactions. That same bacteria, when present in healthy amounts, also accounts for as much as 70% of immune function; therefore, its destruction can desimate much of immune system capabilities.
There are multiple underlying causes for dog skin sores including parasites (fleas, mites), poor grooming, dog skin infection (bacterial pyoderma, fungal), endocrine diseases, skin or hair follicle disorders disorders such as seborrhea, medication side effects (from corticosteroids) and allergy.
Various types of sores can appear anywhere within the mouth, including the inner cheeks, gums, tongue, lips or palate. They can have many causes. Mouth sores can be a sign of a more serious medical condition, including celiac disease.