Sudenly you feel an intense pain in your pelvis and hips, making you fall on the bathroom floor. It's so intense you can't even sream. You try, but no sound come out. Unknown to you, your pelvic bones are breaking down and mend back in the shape of a girl's one.
Once the bones set themselves back in their new configuration, your hips don't hurt you anymore, but your crotch itself seems to feel obligated to compensate by being even more painful. In a daze because of the pain, you don't realize what the strange sensations accompanying this white hot pain means. You're not aware that its your organs slightly reorganizing themselves to let enough place for your male genitalia to change into its female counterpart.
One would think such change goes step by step, but instead your balls simply pull everything with them as they become internal and turn into ovaries, your entire genitalia changing itself you the female kind. Only your penis take a bit more time, still poking out of your vagina and looking like its old self, but not for long.
Once your penis finished its change into a clitoris, your new genitalia pump your body full of estrogen, causing even more change in yourself. You can feel your whole body turning more feminine, but this time the only part really painful is your skull, lower jaw include, and needle like feeling in your closed eyes.
As the pain finally abate, you slowly rise up, not without difficulty, your body feeling very strange, different and your brain is still feeling the aftereffect of the intense pains you felt. Bracing yourself on the bathroom sink, you look yourself to see the 18years old face of your sister where should be your 12years own. At first you think she found a way to enter somehow, the flabbergasted expression on her face… The way she's bracing herself on the sink… No doubt, it's you in the mirror!
Still unbelieving, you ask yourself "I that really me?!?" in your sister's voice. "Woah… At least there is no way her friends won't thing I'm her."
You decide to admire your new self a bit longer when you hear someone frantically knock on the door. Who's on the other side?