I have struggled over the past with my cat peeing outside the litter box. I now permanently put a pee-pad out which takes care of MOST of the problem but not all. I do not think anything has changed which makes her do this.
Is this a behavior or an illness?
There is no way to know for certain whether your cat's urinating outside the box is a behavioral problem or a cat illness symptom. Without knowing your cat's age, possible medical causes include urinary tract infections, metabolic diseases, including feline hyperthyroidism, diabetes, and even kidney disease. Your cat needs a physical exam and urinalysis to rule out medical causes before you can determine whether or not this cat behavior is behavioral. You can read more about cat elimination problems on this page.
All the best,