While a vet may be your best option, as Skitch said, if that is not an option (and if you're a pet owner, it should be!) then here is what my vet told me: Sometimes, cats get sores on the upper (roof) of their mouth. That makes it hard to eat.
He didn't say how to repair the damage, other than it can lead to worse sickness and possible malnourishment and other illness. He suggested adding more water to our kitty's food, even the soft food: Make it almost like a thick soup, he said, so it's very easy to "drink" for the cat. Soften it up and make it more liquidy.
If it continues more than a week, you really need to see or call a vet, ok? And good luck to you and your cat.
