Question on what is going on with my mother?

So my mother started off about a year ago with minor memory lost and anxiety in the evening. We looked it as getting older (71) and laughed about sundowners. As time has gone on she has gotten worse. About 4-5 months ago she called me in the middle of the night telling me there was a man in her house and she didn't know what to do. I immediately called my dads cell and he answered, turned out she was speaking about him and this is when we realized something wasn't right. The forgetting my father begin to increase quickly and more frequently. She has been getting angry, annoyed easily (her trigger) and very paranoid. Once she gets trigged she started to get confused. She has seen a neurologist, got a MRI and a EEG but they came back with no signs off Alzheimer's. She has a appointment with a neurology group to see if she qualifies for a trail, where she will get a Amyloid PET Scan. Her neurologist said this will help see better what is going on. So I pray she gets accepted. I guess the point of this message is to see if anyone has any information. Why is it happening so quickly, so often, why only my father, why is she seeing "people", how can she not know my dad that moment but know everyone else, talk to us like she about this "other person" like we know them? It so mind boggling.