I see this question frequently and as a person with some problems myself, I may have a partial answer.
For me it’s adrenaline.
sitting around the house or in the car, watching most television, or being quiet, my mind goes to the rest mode. Sometimes even in the store or doctor’ lobbies or in the small room waiting for the doctor.
Talking to the doctor or nurse I feel alive, with it, on top of it.
I have only moderate problems so far and I know that people with more severe problems wont always act the same but if mom or dad or whoever suddenly perks up and is no longer the same person that you know at home, adrenaline might be the answer.
best wishes to you. And thank you for all you do.
We are quite determined to "present our best face" to strangers.
We can often act much better than we feel or than we "are" for brief periods.
Because that adrenaline is present (you are right about that), we also don't feel pain.
Adrenaline coursing through our body released natural endorphins which are in fact pain killers.
Get home and relaxed and it all goes achy.
As to the dementia patient who is suddenly so chatty, bright and well-seeming. Confabulation allows for the "social niceties" to emerge and take over our norm. We are suddenly all "Oh, so good to SEE you, Dr. P. How are your children." and blah blah. A doctor with 10 minutes on the docket to see you can often be fooled though they are now TRAINED (because MEDicare demands that patients are evaluated for mentation yearly to get Medicare payments) to override this confabulation. They may ask a test question you don't even KNOW is a test. Such as "Tell me again what pills you are taking, Irma?" when of course the pills are right in front of them on the screen.
The elder who comes back from Doc and collapses with a thousand complaints doesn't have the adrenalin feed. In fact they are a bit shocky and a bit post traumatic stressed. Now everything is confusion and pain.
That's called the "norm", hee hee.