Alzheimer disease is only one form of dementia but because it is the most common, the various types are often lumped together under the one name, similar to calling all brands of tissue - 'Kleenex'. Each type have similarities but have differing symptoms, behavior patterns and speed of progression.
A PET scan can give some indication to the dementia type but may not be definitive and the procedure maybe upsetting to the patient. As there is no treatment procedure for any type of dementia, a scan is not often bothered with.
Elderly people are also susceptible to urinary tract infections which can cause extreme changes in behavior, which include often severe confusion and forgetfulness and so may appear to be a dementia condition. Behavior change can be brought on as suddenly as turning on a light switch.
Having been a caregiver to a close relative with dementia, I can relate to Rebelfighter's and your experience and offer my respect, sympathy and condolences to you and all those dealing with these illnesses.