Porous Memory - Lost Story

Since my main computer died awhile back I've been a bit lost, and my porous memory isn't helping. There was a multi-part story awhile back, by one of our top authors. Set in medieval time, it's about a boy who is so scrawny he can't find anyone to apprentice him except the local dressmaker. He goes to a tournament and the victor in a duel on horseback chooses him, thinking he is a girl. As the story goes on he actually turns into a young woman, and what a little hellcat she is!

I know everybody except me knows the story and author by heart, but my brain keeps sending back 404 messages. HELP!!!