Hello, Kim, and welcome to TP. There is lots of good advice and support available here and I hope you will find some help and relief. Don't forget that TP is always open!
As you have heard from others, you are not alone in experiencing this. My mother (73, Alzheimer's, no short-term memory) definitely went through a bad patch of this kind of behaviour. She was only nasty to me, and only when she thought she couldn't be overheard. She would stop in the middle of a vitriolic rant to sweetly thank someone bringing her coffee, and chat with them very appropriately, and then switch right back to laying into me. It was worst while she was in hospital (under a section) and then right after her move to the care home.
It was, in fact, so bad that I didn't visit her for about two months. Then I started short visits, never alone, and would leave the room at any sign of unpleasantness (I had to go to the toilet a lot). Things did improve and we are now at the stage where there hasn't been a nasty comment in person for months.
My mother's neurologist talks about these kinds of moods, and suggests that if possible, you use humour or a distraction or a change of subject or a change of location, to help "flip" their mood back to something more positive, but acknowledges this isn't always possible.
You could try this. You could also think about how you could change the structure of your visits, to make it less awful for you. Shorter visit, different location, offer an activity, offer a cup of tea, go with company, whatever.
How often do you visit, and for how long, and is it usually on your own? Where is your mother living? If she is at home, does she have carers come in, or go to day care or a lunch club or something similar? If she is in a care home, have you talked to the staff about this?
Personally, I would try leaving the room when she starts, and then beginning another subject when you return. If the verbal abuse continues, I would simply end the visit. If you are never able to visit without this happening, then I would consider not visiting for a while. But that is just what works for me, and your experience may differ.
This factsheet about dementia and aggression may or may not have some helpful information. (The Alzheimer's Society website has lots of good information and resources on a variety of subjects.)
https://www.alzheimers.org.uk/site/scripts/documents_info.php?documentID=96
It's very challenging and unpleasant and I am sorry. Best wishes to you.