My mother goes from lucid periods to "dementia" in seconds. Most of the time shes in a dream-state.

I agree as I so often do :~) with Mrranch2. I'm both the carer of someone with dementia and now have it myself too. I'm still comparatively early stage but I know I can constantly forget things, but if I think someone is cross with me that lasts.
It's not so much that I remember the issue or what caused it, it's more that I'm still aware of us being out of sorts. It's as though I feel the emotion in the air but I can't remember why it is there.
What I can tell you is she's not faking and she's not trying to upset you on purpose.
Hope this helps.

Very complex to try to explain (we patients have a strange way of looking at things--try to never argue with us, our reality is different from yours) but sounds like she's remembering the emotion rather than the incident.

She is remembering your being upset with her (even if you weren't, she thought you were--I feel that way with my wife quite often and feeling guilty for the same reason). I think your mother may be doing the same thing, just trying to make amends. Think about children; they quickly apologize if they think they made their parent angry or if they did something wrong, and they usually don't let it go until the parent reassures them.

Don't fret over it or discuss it further, just hug her and tell her its ok--even if that's a fiblet--and that you love her, and that may end it (it works with me). Worth a try.....