Dog snaps when head touched.

Dear Dr. Marie,
Our 7 year old bichon frise has started new concerning behavior. Her background may be important and I will try to be brief. The poor thing had Calcium oxalate stones that were removed at age 2 and she has been on Hills W/D diet since. We monitor her urine frequently. She also has luxating patella. From the day we brought her home from a well researched breeder, at 12 weeks, she has been sweet and treated with love. Only positive reinforcement training. However, she has never cared to be held or approached to quickly. We have always been respectful of her ways and over time she became more affectionate and loving. She does have many fears. She has always snapped if you touch her while she is sleeping. We learned to speak to her first and let her know we'd like to pet her, then she is fine and loving. She often sits next to me at the end of the day for a back rub and ear massage...her favorite. She just loves the ear rub and rolls into my hand and just enjoys. Each year becoming more relaxed and secure. For the last few weeks she has suddenly started to snap at me in the middle of a head rub or ear massage. So strange. I'm afraid to have her lay on my lap or sit next to me to be rubbed as we have done for years because of this. She just had her physical and is healthy. Her urine had a bit of bacteria so it is being treated with an antibiotic. She takes Denamarin for high liver numbers that she has always had...565 this time. All other blood work is normal. Ears checked and fine. Everything else is status quo. I don't understand and want to help her and give her love and affection but I am afraid she'll bite. She was checked for cushing's last year and an ultrasound was done on her liver ... all was well. She is also on Potassium to keep the crystals out of her urine. Frontline and Interceptor monthly. I do think some arthritis may be starting. Help? Thank you, Susan Montanaro