Actually, I do think your suspicion is right that there is a problem. Even something like a UTI can cause short term dementia in the elderly. Or it could be a more long term problem. If her doctor will not listen to you, it might be time to find a new doctor. Perhaps one that specializes in elder care.
When my grandfather was developing dementia from Alzheimers disease, before my grandmother finally sought help, he did some scary things. Like before his car keys were taken away, he went out to drive to the store, then forgot why he was driving. But things started out much more benign, with forgetting 'simple' things. And I know it can be hard when you are getting frustrated, but patience is key. It is often frightening for someone if they realize they are forgetting things, that they don't have the same mental grasp. It means less independence, too, and that is very difficult for people to adjust to.
My grandfather did not get help until my father forced it on him, by becoming his power of attorney. You see, my grandmother kept denying the problem, and the situation was getting worse and worse. I think perhaps if he'd gotten help early, he would have had a much better quality of life in his last years. So it is good that you know something is amiss NOW so she can be treated. And it may be something short term, only a medical professional can figure out what is going on.
