I'm sorry to hear of this with Kelsey. The etiology of anorexia is exhaustive (I'll post it below.) but when seen in a 12 year old, time is of the essence. A dog of Kelsey's age doesn't have the reserve of a much younger dog and will become dehydrated and electrolyte imbalanced quickly. This is the stage of life when we more often see degenerative and neoplastic (cancer) disorders in our dog. Chronic renal or hepatic failure is relatively common; both will cause anorexia and should also cause her to be drinking more than usual. While her anorexia is an important symptom it's not pathognomonic (specifically indicative) of any one disorder. I encourage your having Kelsey's vet perform a thorough physical exam on Kelsey including a senior/geriatric diagnostic panel of blood and urine tests. Imaging in the form of X-rays or ultrasound may be recommended predicated upon the finding in her physical exam. I understand that you didn't come to this site to be told to take Kelsey to her vet but that's the best advice I can give you at this time. Please respond with further questions or concerns if you wish.
Anorexia is one of the least specific clinical signs and will not in itself direct the clinical evaluation. Anorexia is purely an indication of underlying disease. For the truly anorexic patient, the causes may be legion. It's vital to differentiate between a patient disinterested in eating and one who experiences difficulty or discomfort while eating. Thereafter, routine laboratory tests and diagnostic imaging complement the history and physical exam for elucidating the cause of anorexia.
Here is a synopsis of the differential diagnosis for anorexia:
Disinterested in food altogether (true anorexia): systemic disease, infection/inflammation, neoplasia, food aversion
Reluctance to eat (pseudoanorexia)
Associated with pain/discomfort
Painful prehension or mastication, odynophagia (repeated attempts at swallowing) as seen with: retrobulbar abscesses from apical/tooth root abscesses, e.g., intraoral masses/foreign bodies, mandibular fractures/temporomandibular joint disease, masticatory myositis (inflammation of the muscles of chewing), periodontal disease, salivary gland disorders, oropharyngeal dysphagia, esophageal disease (masses, foreign bodies), nasal disease affecting sense of smell.
Associated with nausea
Gastrointestinal inflammatory disease Ileus (paralysis of the GI tract), delayed gastric emptying, vestibular disease, side effect of medications; many drugs have GI tract side effects.
The initial database for pseudoanorexia is a neurologic examination, oral, dental, and cranial examination (sedation or general anesthesia); radiographs of the teeth, mandible, or nasal cavity may be required. Thoracic radiographs and/or endoscopy to evaluate the esophagus and gastroesophageal sphincter for any obstruction (e.g., strictures, masses, foreign bodies) or mobility problems. The initial database for true anorexia involves laboratory testing and imaging as suggested by history and physical exam findings.