You were in a family that transforms into feral animals. It doesn't happen often, because its always caused by a full moon, every time you notice it. Every time the moon is out, you become an animal that gives you a massive sexual urge whenever you see that full moon. It's also a complete waste of clothing every time you transform.
The moonlight causes your entire body to grow a bunch of fur that covers your face, chest, and limbs, you sprout a tail, your genitals change to match that animal, and you lose the ability to walk on 2 feet. It happened a year ago, when you were walking one night, and a mysterious creature bit you from the bushes. You managed to survive, while at the same time getting it looked at, but was unsafe when you feel the transformations, and sexual urges take effect during a full moon.