Nobody's Prize by Esther Friesner

In this rousing sequel to Nobody’s Princess, young Helen of Sparta is not about to be left behind when her older brothers head off to join the quest for the Golden Fleece. Accompanied by her friend Milo, and disguised as a boy herself, Helen sets out to join the crew of heroes aboard the massive ship known as The Argo. 

Helen quickly faces all sorts of danger. Not only does she have to avoid her brothers’ detection, but a devastatingly handsome boy catches her eye, Hercules falls in love with her boy-self, there are battles to be faced, as well as a terrifying murderous princess and the start of her period. And that’s only the beginning! With her beauty blossoming, Helen’s journey takes her beyond the mythology of the Golden Fleece to Athens, where her very future as Queen of Sparta is threatened.