Yes, 'why' indeed. When it comes to the workings of peoples minds, I guess there could be a whole plethora of reasons why she would say/do those things, so, at the end of the day, I don't know why.

There's the possibility that right or wrong, she honestly believes you didn't use the toilet roll and so asked you later on to have another shower, for the personal hygiene. Maybe she forgot she replaced it? But if she did in fact replace that roll then lied to you, well, that's just really weird, to me. It's a bit twisted.

Maybe just let it go this time Bookworm257, notch it up to a misunderstanding, a mistake, on her part, and just see how things go from there.

Also, if it's possible, call her out, in a very calm way, on any future gaslighting. Say something like, "Please don't gaslight me, it's not right." And hopefully a peaceful discussion can eventuate from there.

All the best.