Possible mental issue with dog
My 6 year old shih tzu x chihuahua has renal dysplasia and chronic pancreatitis. For the last 6 months she has been acting very strangely on and off. Once or twice a month, she will suddenly stop eating completely, refuse all treats and table scraps, and become very restless (pacing and trying to leave our yard). This behaviour will last 2-4 days and then suddenly return to normal. She usually loves food (especially treats and people food) and stays very close to us (she is always an off-leash dog and we can trust her).
The first time this happened, I took her to the vet and had every possible test on her. Aside from losing weight because of not eating for a few days, everything came up normal. Blood results were normal, indicating no flare up of kidneys or pancreas. The vet couldn't find any physical reason why she would be behaving this way, and suggested that it could possibly be something mental, but didn't go into any more detail than that. I am wondering if anyone could offer any kind of insight as to why she lapses into this strange behaviour every few weeks.
More background: she is currently on a homemade diet of cooked ground turkey and brown rice mixed with a pumpkin/apple/cinnamon sauce and a small amount of canned dog food just to make the taste more appealing. Fat and protein are kept relatively low and of good quality to prevent aggravating her health problems. Her medications are calcitrol, pepcid a/c, aluminum hydroxide, and a powdered probiotic. She weighs about 8 lbs. This issue had never occurred before she turned 6.
Any suggestions at all would be greatly appreciated. I used to work as a vet tech, so don't be shy about using medical terms. Thank you!