If you are feeding or have fed your cat commercial cat food, sometimes it
doesn't have enough nutrition to it , especially if a cat isn't feeling well.
Try giving your cat brewer's yeast tablets- she may eat them out of your hand,
cats like them that much( or grease them with a little butter and pop them in
her mouth if she won't take them and you're adept at giving pills) , as
well as Vitamin E(natural form not synthetic) about 25 IU's a day. See if
your cat will lap up a tablespoon or so of flaxseed oil too. If, after a
couple of days, the lack of appetite still persists, see if your vet will
give you something called Cyproheptadine, it stimulates flagging appetites.