@Fragglemuppet there are people in my family who have undiagnosed psychiatric disorders and are very compelling storytellers. Everyone was very sympathetic to them and wanted to help until we started to discover that we were turning up in their stories to other people as the cause of their problems, sometimes in monstrous ways.
In one case, I confronted some of these other family members for their years of abuse and bullying only for them to confront me right back about my years of abuse and bullying.
That's when the lightbulb clicked on.
@MisuseCase schizoaffective disorder is exactly what I think one of my family members has. Innocent interactions from today will morph into something completely else when they're recalled from memory the next day. As with your mom's experience, any negative emotions they have gets confused as being produced by that completely unrelated interaction.