Please take your kitty to the vet and have a blood test done. As hissy pointed out, this is often a sign of anemia, and it can be very serious.
One of our kitties was eating litter. Our first vet said "Oh, it doesn't mean anything." We got a second opinion. "Oh, it doesn't mean anything."
Several months later and our kitty was almost dead from severe anemia. His is due to an autoimmune disease. We (and he) fought for two years to save his life, and he came very, very close to losing it twice.
Cats can usually receive a blood transfusion only once. Their blood is more complicated than human blood, and after that first transfusion their bodies identify the "new" blood and it coagulates - preventing transfusion.
So please do not wait until your kitty's gums are white instead of red and he's just lying around doing nothing, because at that point it might be too late to save his life.