this is a sign of an anemic cat &/or calcium imbalance .....get to a Vet and get a blood test asap for your kitty. also switch to a paper based litter if you are using scoop-able clay little b/c this expands when its wet and will swell up in kitty's abdomen and cause an intestinal blockage....very serious!! heres a write up
http://www.felinecrf.org/anaemia.htm#eating_litter
Clay cat litter? Or is it some organic substance, such as ground corn cob? I would suspect a vitamin deficiency, but would talk to the doctor.