Is this a sign of final stages of dementia or is it just something "normal" dementia patience experience? Her vital signs are good or can someone die even though their vital signs ie; blood pressure, pulse are good? Can they work themselves up to have a heart attack or heart failure?
There is no 'routine' death experience, as each person is unique. From my long term care experiences as a nurse -one person could be acting quite normal and bam- dies in a matter of minutes. ( ...as was my first death experience with a resident.) Another person, food begin to 'taste different' and there is a gradual decline.
Vital signs can be quite normal. The heart can keep beating for a bit after someone actually passes. Caregivers and doctors can only go on external factors, and are often wrong in death predictions. Some experienced staff have a kind of 'knowing' of death approaching in a resident... but not every time.
( Interestingly, animals often sense approaching death better than humans can.)
I have seen people on 'death's door' hospice be taken off hospice for a while, then back on again.
People can rally for a few months or they can hang on by a thread with very high body temperatures for days on end.
With dementia, the thought process is affected, and yes - it is possible to work oneself up with agitation, but not very often to giving oneself a heart attack - unless there is that underlying factor present. Though, stranger things have happened.
From my experience, some people just 'know' that their time is approaching soon. Others seem to be taken more by surprise, as it were.
I would take this as your mom's way of saying good by. It is a precious gift. It might not be long... or it may be a long time. Only God knows when her time will be. It can be a very emotionally exhausting for you, though... expecting it.. but the event not happening. You will look back some day with 20/20 vision and be able to put all the signs together better. This is a difficult time for you. You have my concern ... sending you a hug.
Hope this helps. It is a difficult journey for everyone.