Is my nan suffering from some sort of dementia? ?

My nan is 90 years old and very independent. My concern is a few years ago she caused a major rift in the family by accusing my niece ( her great grand daughter) of taking twenty pounds from her room. My niece is a very gentle girl and I'm 99.9% sure she wouldn't have done it. Now, she's had workmen in the house and believe they have stolen 60 pounds from her purse, even though there was 200 pounds in there for her shopping. She then accused another workman of stealing 2 cups from a box, because she can now close the box when she couldn't before. Now just last night she believes someone had stolen the workmans wheelbarrows from her back garden. I don't want to accuse my nan of losing her mind but I Dont wholly believe her, and I don't want to run in accusing the workmen