Tobin, the title character of Lynn Flewelling's "Tamir Trilogy" (there's a name change later; just go with it), suffers debilitating cramps and discovers blood in his trousers when he's about thirteen, and runs away from his friends because he thinks he's dying of the plague and doesn't want to give it to them. Nope, it's just The Awesome Power Of Menstruation breaking through the spell that's given her the shape of a boy since birth (a shock to her, but not to the reader). Given the amount of page space devoted to menstruation as the books progress, women in Tobin/Tamir's world must get their periods about once a week.