She thinks she's "done something bad"

My mother has later stage Alzheimers dementia. She sometimes recognizes her children but doesn't know our names and requires assistance with all of her ADLs. We care for her in the home she has lived in for over 60 years. She has recently been perseverating on "I've done something bad" We can't find any pattern or association with time, activity, location, etc. We try to distract her when this occurs but it is hard to break her out of this and she becomes tearful. Any suggestions for why this might be happening and how to better manage it would be appreciated.