Two things to consider, I think:
1. You should definitely have a doctor see her and have some tests done. It could be something like multi-infarct dementia which involves a series of small strokes that basically go unnoticed but still can cause minor brain damage. It started to happened to my dad in the last year or year and a half of his life. He was pretty much mentally sharp until a few month after about 80, though he wasn’t quite as sharp as he once was but he went from being a man with a Ph.D in physical chemistry and a pretty good chemist to a man who couldn’t figure out how to make instant coffee in the microwave. My metaphor for it was that he was kind of of like a Christmas tree and every once in while one of the lights would flicker on and off and… then off. Forever. It’s important to find out what’s going on with her. There are different kinds of senile dementia with different treatments.
2. You should also consult with a doctor about your grandmother’s medications, what she’s taking, what combinations and dosages, and any changes in her medications that may have occurred when this started happening. My grandmother also had some cognitive problems that turned out to be medication related or problems that were at least exacerbated by her medications and were alleviated by changes in her medication. She did go on to have full blown Alzheimer’s years later but changes to her medications helped a lot.
Anyway, see if you can get her into a doctor for an evaluation and keep an eye on her.
That’s my two cents at least.