A twisted ovary or testicle can be spontaneous!
You probably felt some physical pain just reading that sentence. Ovarian or testicular torsion is when the ovary of the testicle (respectively of course) gets twisted on its own ligaments. It can happen completely spontaneously. One minute you can a happy, healthy person with a normal reproductive system and the next minute you could be writhing on the floor.
This condition requires emergency surgery to save the affected organ, prevent relative sterility, and end the nauseating pain. The lower abdominal pain felt as a result of ovarian or testicular torsion is thought to be equally painful as labor contractions in the transition stage.