Hi!
First of all, go to the veterinary with your cat, sometimes this change behaviour could point some health problem as you could read bellow:


Feline Behavior Problems: House Soiling

House soiling is the most common behavior problem reported by cat owners. It includes urination and/or defecation outside the litter box, as well as urine spraying.

Why do cats eliminate outside of the litter box?

One common misconception is that cats soil in inappropriate places for revenge. It is tempting to conclude, "He defecated on the living room carpet to punish me for leaving him for the weekend." But this kind of calculation requires sophisticated cognitive abilities that cats aren't believed to possess. Furthermore, this conclusion assumes that cats view their urine and feces as distasteful, when in fact they do not. It is only we humans who view it that way.

Medical Problems
So why do cats urinate or defecate on your bed or carpet?

Medical problems are one possibility. Inflammation of the urinary tract may cause painful or frequent urination, inability to urinate, bloody urine, and crying during urination.
An affected cat is likely to eliminate outside the litter box if he comes to associate the box with painful urination, or if he has an increased urgency to urinate.
In addition, kidney, liver, and thyroid diseases often lead to increased drinking and urination. Inflammation of the colon or rectum, intestinal tract tumors, intestinal parasites, and other gastrointestinal conditions may cause painful defecation, increased frequency or urgency to defecate, and decreased control of defecation.
Age-related diseases that interfere with a cat's mobility (for example, arthritis, nervous system disorders, or muscular diseases), or with his cognitive functions can also influence his ability to get to the litter box in time. In short, any medical condition that interferes with a cat's normal elimination behavior can lead to house soiling.

Litter Box Aversions
Behavioral problems, such as litter box aversions, inappropriate site preferences, or urine spraying can also lead to house soiling. An aversion implies that there is something about the litter box that your cat finds unsavory. It could be the box, the litter, the location of the box, or all three.

Something about the litter box bothers your cat.

The box contains harsh odors. The litter box may have an offensive odor if you clean it with harsh chemicals. Or, if you don't clean it enough, the box may smell strongly of ammonia (a normal byproduct of urine). In either case, covered litter boxes hold in and amplify such odors.
The sides of the box are too high. Cats with painful legs, sore joints, or other mobility problems may have trouble getting into a box with high sides. Kittens have similar problems.
Something about the litter bothers your cat.
The litter is dirty. Cats usually prefer clean litter.
The texture of the litter is distasteful. Your cat may have a preference for finer-textured clumping litter over coarser non-clumping litter—or vice versa.
The scent of the litter is unpleasant. Most cats prefer non-scented litter.
The location of the litter box bothers your cat.
The box is in an unpleasant area. Avoid placing the litter box in a high-traffic, noisy, dark, or dank area.
Your cat is afraid to use the box. If another cat, dog, or human terrorizes your cat when she's in the box, or ambushes her as she exits, she may avoid the box altogether.
Cats with aversions usually eliminate on varying surfaces. You may find puddles of urine and/or feces on either soft surfaces like carpets, beds, or clothing, or on hard, shiny surfaces like tile floors or bathtubs. Depending on the severity of your cat's aversion, he may continue to use the litter box, but only inconsistently.
...

for more information, please consult:

http://www.vet.cornell.edu/fhc/resou...sesoiling.html

best regards