You were competing with one of the dukes and the condition was that if you lost, you would have to be the duke's bodyguard.

You lose and you have to be Duke's bodyguard...

One day the young duke enters your room and sees you naked and realizes that you are a girl.

"Wow.... I should have known you were a girl... How did I not understand from your short height.... Seeing your body, he orders you to sleep in his room tonight as his lover"