There are several reasons for a cat to urinate outside the litter box. She may have a bladder problem such as an infection or crystal/bladder stones. These cause her to be uncomfortable and intermittently avoid the box. This may also be a behavioral problem such as not liking the litter or being stressed (company, new pet or baby in house, stray cats outside….). The first step is a thorough history (specifically what is she doing at home), paired with a thorough exam and evaluating a urine sample.
Christine Simon D.V.M.