Sounds like she's a liar, frankly, and was just looking for an excuse. Not every old person who creates a lot of drama has dementia. There are plenty of them with minds that are fully intact and they're just assholes.

Either Grandma has a bad temper or she may be moving into dementia. Chances are by now she has gotten over the episode and is ready to forget about it. When I was a boy I idolized my grandfather. I liked helping him whenever I could. One day he asked me to help him carry a full sized cured ham up to his storage building. Unfortunately on the way I tripped and it hit the ground. Grandaddy said “You little devil.” I was destroyed. I did not do it on purpose. He was able to salvage the ham. I cried later. But Grandaddy got over it, and the next day it was as if it never happened. My Granddaddy was simply temporarily overcome by frustration and emotion - As was your Grandmother. Unless your Grandmother often acts like this, don’t let it bother you too much. Offer to replace the glass if you can.