Your legs grow scales.
As they do, you feel a tail rip through the seat of your shorts.
Wings start to grow out of your back.
As they do, your tounge becomes forked.
Then all of a sudden your chest bulges out.
Your groin changes as well.
You realize that you are now female.
The scales continue to grow and soon cover your entire body.
You feel your head reshaping as the dragon's horns make their presence felt.
As you look at your hands and feet, claws poke out.
You feel your body making the final changes and, all of a sudden, you start to grow.
You look into a nearby puddle of water and realize that you are now a dragon.