What Could Cause A Minor Memory Loss In An Aged Person?

My mother is 89 and in the last week her mine is every where. She can t remember to take her pills. She doesn t know if it s morning or night. She has pain in her hip. Whenever given her pain meds she denies that she gets any relief. Could the pain be so bad that nothing gives relief or is there an underlining issue? My 2 siblings are away Snowbirds, and it seems every time they leave she always gets some illnesses. The one is the worst because it is scaring me as she is acting so different she can t remember one minute to the next. Does Dementia start suddenly like this? Her mother lived to 94 and did have. It. She cries a lot and keeps saying her brains don t work. I can tell her one thing and in 2 minutes she forgets and I am repeating it all over. Thank you. I await you advice