What's the sore like? How long has it been there? Is it like a lump, or an open wound? It's hard to know what it is like, from what you said. Obviously, if it's been there a while, and won't go away, go see the vet.
My dog had a little hard dark irregular-shaped lump on the outside of her lip. The vet thought it was suspicious, so he biopsied it. It was not cancer, but an infected hair follicle.
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