This is likely the result of enlargement of the mandibular lymph nodes (the nodes on either side of the neck just below the jaw). This can be due to a number of causes, all of which should be checked out by a vet.
She may have an infection (skin, teeth, ears) that is draining into the mandibular lymph nodes. Your vet will be able to determine this and get her started on the appropriate antibiotic or ear medication. Some inflammatory processes can cause enlargement of the lymph nodes as well, whether it be the result of an allergic reaction or her own body attacking the lymph tissue ( as in systemic lupus- a disease in which the body's own immune system attacks its own tissue).
Less common, but not unheard of in a dog her age, would be a type of cancer called lymphoma. This causes the lymph nodes to become enlarged and firm; often multiple sets of lymph nodes can be enlarged and the dog initially seems fine.
Your vet may elect to do a fine needle aspirate of the affected nodes to look for evidence of infection, inflammation, or tumor.
I'd recommend giving him a call and having him take a look at your dog as soon as you're able.
Edited by Vet help on March 27 2006 at 2:31 PM