My mother is 81 and lives alone. She lives 40 minutes away and no longer drives which is another conversation all together. She is getting a ride from a friend to go grocery shopping nearby. Then she was going to be dropped off at my house. We were then going to dinner with my family and then I was going to take her home. She thought we had a conversation about her staying with us that night and then I was going to take her shopping, to church and then home afterwards. That part of the conversation never happened and now I'm probably as stumped as she. She has not been diagnosed with dementia but she has had some short term memory issues with some cognitive decline over the last 3 years. Could that conversation that she thought we had be related to dementia?
Have you been to her home lately to see what's going on in there? Is it dirtier or disorganized more than normal? Is there unopened mail? Or bottles of opened pills lying around? These are very common signs of a problem.
Another reason to get her in to the doctor is to make sure she isn't having a UTI, which, in the elderly, often presents symptoms that mimic dementia, like confusion, delusions, hallucinations, agitation, etc. This is testable and treatable. If left untreated can become septic and dangerous. Best to discount all other medical problems before coming to the conclusion that she has the beginnings of dementia and memory impairment. If you have siblings, it may be time for a discussion on how best to help her.