Is sensitivity to brushing of teeth a typical dementia problem?

My 88 year old mother has Lewey Bodies Dementia. She lives with me and my husband; I am her primary caregiver. Lately, she has become very sensitive to the feel of the toothbrush as I try to clean her teeth. I have switched to a "toddler" toothbrush which is as soft and small as you can find. It started out being only when I brush behind her teeth that she seemed bothered, but now anytime the brush even touches her gums she says it hurts. I am brushing so slowly to avoid the "pain" that I doubt I am doing much good. Anybody else have this issue? How do you keep their teeth clean and breath not rancid?