Is a quick loss of train of thought or forgetting what words to use a sign of dementia?

Absolutely get her checked out with an MD.Sudden changes and by that I mean happening right in front of you could be signs of a stroke.Only way you can be sure is get her checked out and as FF said if this is happening right now please call 911.

I am not sure if she has Alz/dementia. I was just wondering if these were signs leading to that.

it has been a slow progress, that this is happening to her.

Not to make light of the OP's query, I'm in big trouble as that happens to me frequently.

Sunflower, your profile states that your mother already has Alz/dementia -
is this a new, abrupt change? I also thought of a stroke, in which case the onset of symptoms could be a warning sign. But it could also be a new stage of the Alz.

Whomever is having that issue, get that person to the ER or urgent care quickly. It could be a sign of a stroke.