Behavioral or Illness?

Question
I have a Seal Point Siamese, female, 9 years old.  She is only about 4-5lbs, she has always been small.  Recently, she both urinated and defecated on our bed which is very unusual.  She has never done this outside of her box.  She is ordinarily a very well behaved cat.  I am unsure if this is a behavioral problem or if she could be sick.  I have heard that cats hide there illness very well.  This has only happened a couple of times within about two weeks of each other.  Any advice?

Answer
Yes cats do hide their illnesses.  I would take her to the vet to have a urine sample checked.  Failure to use the litter box is often due to a medical problem, and we only determine a behavioral problem AFTER all medical causes have been ruled out.  I would also recommend getting another litter box in case she is associating the old one with pain.