Sundowning.

I agree with others who say this doesn't seem like Sundowning at all. I've seen it, and it doesn't look like what you describe. Seems like hospital delirium or the post-stroke effects.

Is he on a catheter? Has he been checked for a UTI?

I was also thinking a a TIA or stroke.

From the sequence you described I would not call that Sundowning.

The 3 days of not speaking, confusion & aggitation I would guess to be the effect of the STROKE itself.

Stroke is either a bleed or clot in the brain. It causes injury inside the brain.

After the brain bleed or swelling reduces, then the person may (hopefully) recover function. Recovery varies a lot, depending on severity of the stroke, other disease & frailty.

So, YES, it can improve.

DELIRIUM is a medical term used to describe a fast onset serious change in mental abilities.

Delirum can be caused by many things - UTI, other infections, pain, surgery, medications, changes in sleep pattern & more. Even just being in an unfamiliar environment like a hospital.
YES, it can improve.

'Sundowning' is a term used to describe confusion &/or changed behaviours in the late afternoon or evening. It is commonly reported in those with Dementias.
It may come & go.

I would say the immediate change could be called Delirium, caused by the Stroke.

Whether he has (or will have) Vascular Dementia, it is unknown yet if 'Sundowning' will be present.

Like many things, be aware, be realistic but being hopefully is good too. At 99 he has lived a long life. Hold his hand & share some quality time.

When my Dad had "sundowning", it was like he climbed into a time machine and transported back to the 1940's.

Dad would call me from his senior living facility to tell me he wouldn't be coming home that evening, saying he missed the final bus after the meeting, so he will be staying at a downtown hotel near his office. Dad thought I was my Mom, and he thought his apartment at senior living was a hotel. I always played along so not to upset him.

Cc, as someone else had mentioned, it sounded more like your Dad had "hospital delirium" which not unusual no matter the age of a patient. A young man in his 30's who had major surgery could also get this delirium.

I think you/the nurse may be confusing sundowning
"a pattern of increased cognitive and behavioral symptoms exhibited by dementia patients during the late afternoon and evening hours"
with hospital delirium
https://www.agingcare.com/articles/cognitive-decline-after-hospital-stay-147836.htm

If there was a stroke there are many reasons for changes in mentation, including a swelling that occurs and often resolves over one to two weeks. There is also hospital psychosis which is very common. Dehydration and changes in electrolyte balances can occur. There is often temporary amnesia. That the nurse put this down to "sundowning" means that there are a whole battery of things that may have occurred, but they are likely all unknowns. It is a relief when it resolves on its own. Know that this is not an unusual occurance and what IS unusual is that it so resolved, and so quickly resolved. Hope that helps. This falls into the chasm of the many "unknowns" in medical mysteries.