UTI? Tests clear

[ThinWomansBrain]

Around midnight on Saturday, I noticed my cat (11, female) was straining to pee, scratching at her tray a lot, and squatting as if trying to pee on magazines etc - I've never known her toileting outside her tray before.
She eventually came to bed and settled down, and seemed fine on Sunday morning.
The behaviour started again on Sunday evening.
Monday I took her to the Vet, along with a urine sample (a surprisingly large one given all the squatting with no pee), which he tested, no bacteria, no indication of bacteria or problems with her kidneys, so he didn't think she needed antibiotics & if she had had a UTI it was clearing up. He said to go back for metcam if she still seemed uncomfortable.
Still getting the weird squatting and scratching behaviour now, Tuesday evening.
Any ideas what it might be?

Feline lower urinary tract disease - sterile cystitis caused by a variety of factors, but stress is definitely part of it. This is a great article written by many of the experts in the field.

Did they test for crystals?

@Lonecatwithkitten did you have a link to post - sounds like an interesting read.

Thankfully she seems really calm this morning - and judging from the amount of sawdust in the litter tray when I cleaned it this morning, is back to normal quantities of pee.
I've ordered feliway that arrives today, so hopefuly that will help calm her.

@cinnamonfrenchtoast Not sure what tests the Vet did, but he mentioned no bacteria, lower level than normal of protein (I think) that indicated her kidneys were working well. (.032 rather than .035)