Can a person with moderate dementia turn “sweet and sour” on a whim?

My mother, who has Vascular Dementia, lives in an assisted living facility in their the memory care unit. Last week I took her to the Dr. to get her wrist rechecked (she had broken it recently in a fall), and a couple of other places she wanted to go. When we got to the last place, (the vet, to get some cat food), I told her she could just stay in the car if she wanted. I thought it would be easier on her and it would only take me a couple of minutes. Immediately I could tell something was wrong because she was glaring at me. I asked her what was the matter and she said she had wanted to go in. Fine, great, come inside with me. “No, you don’t want me to!” Long story short, she remained mad at me and we didn’t go out to lunch as we had planned because she decided she didn’t want to be around me when I was treating her so badly. When we got back into the AL building, she saw someone she knew in the reception area and was SO SWEET to that person. My brother says it’s due to the dementia. She doesn’t pull this kind of crap on him, by the way. And she does it quite often to me, the one person in the family that takes her for outings. If she can be selective enough to be sweet to one person, but sour to me, is that the dementia?