She is marking her territory, cats do this
http://www.hdw-inc.com/litterbox.htmThere is a major difference between urine "marking" and urine "spraying" in cats. Both are done deliberately by a cat, and are used to deliver a specific message, usually a message intended for other cats long after the sender is gone. Urine spraying is done by a cat backing up against a vertical surface such as a wall or curtain drapes, and spraying their urine against the surface while in a standing position. Urine marking is done with a cat in a squatting position, causing the urine to squirt onto a horizontal surface. The reason cat urine has such a powerful odor is that there is a fatty ingredient in the cat's urine which allows it to cling to surfaces and objects, and also causes the strong smell. Un-neutered male tom cats have urine that is especially pungent and unpleasant. Contrary to popular belief, BOTH male and female cats can spray and mark with their urine.