Stiffness and lethargy and the change in behavior you have noted all combine to a trip to the vet if it were my dog. It could be something as simple as a bite (snake, spider, some other kind of bug), or it could be something more serious like an abscess, or lyme or some other tick borne disease. Even a "simple" spider bite could become serious. Last year the vet I work for saw several dogs with brown recluse bites...they started similarly to what you described and within a day or two the area became necrotic and sloughed. They were pretty gnarly and most of those dogs still have scars where the bite was.