It was a oneshot extra of another shojo series, but i don't remember what series it was.

A swordsman goes into town and everyone in the town is talking about how the winner of the swordsmen competition will get to marry the princess and become king. Meanwhile, some people are talking about how the whole thing has been rigged so the prince of a big-shot neighboring kingdom will win. The swordsman runs into a small boy that is fighting? and is in trouble and he rescues him, somewhere along the line, he finds out that the boy is actually a girl, and that the girl is actually the princess and she's disguised herself so that way she can enter the competition and win. but she gets sick right before the competition, he enters himself for her, he wins, blah blah blah, and then it turns out that he's the prince who was supposed to win.

Thanks if you can tell me the name of it.

if it helps, the series it was a part of was pretty short, i think only 1 or 2 volumes