Hi There,
You don't mention the dog's diet, if she drinks normal amounts of water, the medication/dose that was prescribed, and if the anorexia ocurred before the medication was started or after.
Her 'arthritis' condition didn't 'just happen', it was 'just diagnosed', so unless it's the medication, I would be concerned that something else may be brewing.
Yes, lack of appetite can be due to pain, but there are many other problems: liver/kidney disease; tumors, urinary tract infections etc, all of which can cause inapptetite. I don't know, but maybe you just went to the vet for a routine physical exam/vaccinations, or maybe for the arthritis............but, now, your dog is trying to tell you something and I would think that a minimium data base should include: a urinalysis; lab work; +/- a whole body radiogragh, (you have a small dog; a split view can be done a a single film). A 13-yr-old dog is a geriatric and annual bloodwork should be performed routinely, as part of good preventative health care. For instance, if her kidneys were showing signs of disese you could switch to a prescription diet before the disease progressed.
I would take her back to your vet.
Good Luck.