Hello, bunnc, and if nobody has said it, welcome to TP.
The compassionate communication is good advice. It's harder to do than it sounds, especially at first, but it does get easier after a while.
My mother's neurologist explained once that delusions (a false, fixed belief) are very common with dementia and that it's almost never a good choice to try to contradict or argue with a delusion. The PWD (person with dementia) can't do logical, reasoning thinking (or not consistently, at any rate) and you will get nowhere. It also can escalate the situation and of course it's no good for anyone, to get upset and have an argument. While the PWD often is not able to remember the discussion or argument, they can remember the feeling of being upset/angry/unpleasantness and that feeling may recur when 1) the same subject comes up again and/or 2) the same person is with them.
That's a long winded way to say, it's always better avoid upset!
A technique that was taught to me was to listen to the PWD voice their delusion, offer some noncommittal, generalized response, then offer reassurance, and then distract. It doesn't always work, granted, but sometimes it does.
So if the PWD has a delusion that someone is stealing their socks, for example, they might say, oh, I can't find my red socks again, those horrible people, they're stealing my socks, I can't stand it, it's terrible about my socks.
If dementia weren't in the picture, I'd say, don't be silly, Mum, why on earth would anyone want to steal your nasty old socks? The carers wouldn't do that. I'm sure they're just lost. I'll help you find your socks. A perfectly reasonable response, but not to a PWD.
To Mum with Dementia I'd say, no, really? Stealing your socks? Oh, my, that's terrible (all with a lot of shocked tone of voice and warm body language). I'm very sorry to hear that someone is stealing your socks. Of course you're upset. Who wouldn't be upset? Anyone in your position would be upset, of course they would. I'll tell you what I will do, I will look into that for you/take care of that for you, and I will let you know what I find out. Okay? Do you know, I'm thirsty (hungry, whatever) and am going to go get a cup of tea/coffee/have some ice cream, shall I get some for you? And then off into domestic prattle, or go for a walk, or put something favourite on the telly, or whatever.
A "conversation" with someone with dementia is always a sort of roller coaster ride and it often feels very surreal. We've had threads here before, about just that. I remember one day we had my mother out for lunch and shopping (she lives in a care home) and she'd been very "normal" that day and I was thinking how nice it was that it was so normal, for a change. Just then she looked out the car window and saw a bird, a very ordinary bird for around here, and said, in a deeply excited tone, "Look! An eagle! It's a golden eagle! Look at it go!" and then we were off on this conversation about an eagle. We don't have those where we live and even if we did, I doubt we'd see one flying around the car park at the local supermarket!
Often I think that acting classes would have been good training for dealing with dementia, that or improv comedy, or both. My mother can't always follow all of a conversation, or doesn't always hear or see things, but she can hone in on any tiny change of voice, body language, or facial expression with scary, laser-like intensity. I can't ever show any kind of upset or frustration or irritation or anything other than calm around her, or she focuses on it and gets upset. I figure I will come out of all this a champion liar or actress, depending on how you look at it.
Sorry for the long ramble, there. I do hope your mother calms down over time. If she doesn't, it wouldn't hurt to talk to the GP and/or Memory Clinic, to see if medication might be indicated at some point. It's also possible there may be changes in routine or environment that might help. Wishing you all the best.