crystals in urine--temporary?

Our seven year old cat recently had her annual check-up and the vet ran
a geriatric panel. He reported there were some crystals in her urine
and the pH was "slightly off". He said in previous years her urinalyses
were all normal.

We had to bring her back a few days later for a tooth extraction, and
the vet performed a second urinalysis. This one came back completely
normal, no crystals. The vet had told us that stress could have
affected pH in the first test--our cat was pretty freaked out at that
visit because, according to my husband, the cat preceding him was
screaming in pain and apparently upset all the cats (and humans) there.
But surely crystals don't form immediately in response to stress, do
they?

She is going back in a month for another urinalysis but the vet seems
to think it was a one-time only thing and her diet doesn't need to be
altered at this point. Has anyone experienced this or know why a cat
could just have crystals in the urine temporarily? Thanks-

yngver