My 9-year-old neutered male cat, Gideon, has suddenly (within the last
week or so) started exhibited unusual changes in behavior and some
symptoms that something isn't right. He will suddenly jump up and
start licking himself -- usually one of his hind legs. He also will
lay there with the skin on his sides and back twitching. Occasionally
he even hisses before licking the leg. I suspected fleas, but haven't
been able to find a single one. He has also grown more listless,
preferring to sleep upstairs in my study, whereas he used to spend a
lot of more active time downstairs in the evenings and would usually
join me on the bed when I retired for the night. And now he almost
always sleeps in or near his litterbox. Yesterday he started having
diarrhea, too. I noticed what looked like fur in the stools. Could
this be something as easily remedied as hairballs?

His appetite seems fine, and he doesn't seem to be in *great*
distress. He hasn't licked or scratched any of his skin until raw or
red.

I took him to the vet last week, and she said it's probably nothing
serious, but that I should watch him (this was before the diarrhea).
She couldn't find any fleas, either. I'm going to call the her again
later today, but I was wondering if anyone here might have any
suggestions as to what this might be. Thanks!