Just keep reminding yourself that her memories of your childhood are likely not very accurate at all so even the whole "swatted your butt once and you NEVER did it again" is likely not what happened at all. My MIL is full of storied about how DH never misbehaved, slept through the night right away, didn't break anything, etc. Well, everyone else's stories are MUCH different, including DH's memories of his childhood. So I think she likely blocked out the really stressful/annoying stuff and nod and smile (and do my own thing).
We took DD on a vacation recently (she was like 28 months?) and after a few days she started having more tantrums too. We had a long drive, ate out a lot, saw lots of new places, etc. And while much of that is stuff she would normally enjoy, she was misbehaving a lot more. I think it was just the stress of the change (even if some of the changes were fun and exciting). She was missing the comfort of having her own stuff and home even though DH and I were there and we tried to keep her routine as consistent as possible.