The burning sensation quickly morphed into an uncomfortable mass inside her vagina. The uncomfortable formation quickly started pushing downwards, and Katie dropped to the floor in pain, throwing down her underwear and prompting Tiffany to look at her lady area to try and see what was happening to her.
Just as the confused Tiff got into a position to view what Katie was prompting at, the younger girls vagina bulged, before parting slowing to reveal a penis head. Katie felt the uncontrollable need to push more and after several 'contractions', she was the proud owner of a not-inconsiderable masculine penis, rooted directly from the core of her very female self.
Tiffany had heard of shemales, which she new were men who wanted to be women, using fake breasts & hormones to achieve this... she had even heard of hermaphrodites, which were people born with male & female genitals... But Katie was now something new - she was still female, but with a penis... a penis growing out of her vagina, completely replacing her primary sex organ in a strange and unnatural (as if anything like it could be natural) way.
As Katie sat up, she saw her new appendage, wimpering and gasping in unison, before daring to touch it in an exploratory manner. She quivered and immediately felt an erection developing, not unlike she felt in her clitoris when she masturbated. Fortunately, as the penis solidifed, it seemed to curl around her body, pointing upwards, almost to attention like a real man's penis. She felt wet, but had no way to actually get wet anymore. Her still-present clitoris throbbed, quaking for attention as the massive organ slid along it, almost causing Katie to orgasm as it went. Her brain struggled to process the new feelings, emotions and changes her body was under going.
She also felt horny, incredibly horny...
Tiffany just stared at her friend, still speechless, confused and (deep-down) quite heavily aroused!