If your
cat is
normally a
social animal, making the rounds of the couches and chairs, reminding you that it's time for dinner, and cuddling with you in bed at night and you notice that he is choosing to nap in the closet instead of eating dinner, and he curls up under the bed instead of on top of the covers, then you might have something to be concerned about.
Any change in behavior that is constant such as urinating more, drinking more water, urinating less, and trying to urinate but nothing comes out (this is a serious dangerous clinical sign of urinary obstruction — see vet immediately), hiding, wanting to be by his or herself when
normally very
social, (a big sign in
cats if something is wrong), off balance, overall weakness, eating less or more than usual, painful crying out.