Is this dementia? Reality check.

Call her doctor immediately. Your mother already had a massive stroke. Now she has had what is deemed a "medical emergency." Echo CountryMouse.

If your mom had a cerebral stroke, it is possible that she now has vascular dementia--caused by strokes in the brain. My late husband had what we now recognize as TIAs (transient ischemic attacks) caused by small clots in the brain, where he became very confused and said he felt a sense of impending doom, sometimes had hallucinations--but nothing was picked up on MRI. Most of the time, he would recover somewhat, but eventually he had a major, fatal stroke.

[Edit: I should have said this first.] Call her doctor.

Your mother has had a cardiac and/or cerebral "event." There was probably no particular sign of it at the time, but you can see its footprints in her abrupt change of behaviours and level of fatigue. I wouldn't say normal, but it is so common that you would almost expect it; and, one might add, your mother's doctors could have done well to point you towards information that prepared you for it. Sigh. They didn't warn me, either...

Clinical investigations done now may or may not tell you anything useful and probably aren't worth the trouble to her. BUT you must get proper medical advice as soon as possible, because it may be that her body is gearing itself up for a Big One that could be prevented. I should pick up the phone now if I were you.

What type of dementia was your Mom diagnosed with? Based on your reports, I'm afraid your Mom's moods are very normal depending on the type of dementia she has. She'll be alright. You'll be alright too: you've got us here to help if you need to vent or ask questions.

It could be a UTI, so best to get that checked out first. In my experience of looking after my parents who both have dementia, changes such as you mention can happen quite quickly. However, they can also revert back again and be fine for a while before deteriorating further.
There comes a time when time is of the essence regarding placing the person with dementia in AL or a Nursing Home as it really can become too much to handle and affect your health as carer. You have to be strong concerning the transition as well as it will not be easy for her to make the adjustment and it may take her some time to settle.
Eventually though things settle down and you can visit without feeling stressed out.
My mum is sleeping more and more. She has periods where she really deteriorates and can hardly walk, has falls and finds it difficult to feed herself and I think she is nearing the end, then lo and behold, she begins walking fine again, manages to eat food herself etc.
It is a strange disease and a cruel one.
Good luck and I hope I have helped.

Have her checked for a UTI will cause these changes.

I was watching my grandson when I had to take Mom in. He was 20 months, she was 85. It was only 2 weeks till daughter found a daycare but it was like taking care of 2 toddlers. I would have preferred my Gson. I put Mom in daycare just to get a few hours to ourselves. They picked her up and dropped her off. Fed her breakfast and lunch. Showered her for me. It was nice. Eventually, I placed her in an AL.

My understanding is that change can be slow over years, but it also can be quite quick. Has your mother been checked for a UTI? Has there been an event that might have stopped her masking symptoms that were already there earlier? Before she is diagnosed as not legally competent, do you have all the paperwork signed and in place – HIPPA, POA, will etc? I hope that someone will give you details about rates of change for different types of dementia. You have to wait and see how she will handle changes - there is no point in worrying yourself now. Courage and best wishes.