Sid's clothes felt really uncomfortable against his skin all of a sudden, and he decided it'd be best to try and take a look at himself and work out what this feeling was. He raced to the bathroom, his face seeming to tingle with sensation as he went, and once he got into it and saw his face in the mirror, he realised it looked totally different. His cheeks looked softer, his lips were puffier, his jawline was softer, the bulge of his Adam's apple had vanished, and his neck had narrowed. His goatee flaked off and landed in the sink as the lower half of his face became almost hairless.
A gurgling in his stomach as well as a deepening of the odd sensation caused him to wonder what was going on down there. He pulled off his shirt, and watched in amazement as his torso literally changed shape before his eyes. Its wide firmness vanished as it slanted inwards to form soft curves, and his shoulders tapered inwards while the muscles of his arms shrank into a more lithe shape, even his hands becoming more small. The dark hair that liberally coated much of his shirtless body either thinned or vanished too, and the texture of his skin felt more smooth.
But none of that could compare him to what came next. He panted in his softer, more high-pitched voice as the tingling sensations accumulated around his chest. What was this? Everything there felt so so sensitive! The sharp cold air making his nipples perk up was more noticeable than it had ever been in his life. Breathing heavily, Sid saw and felt his nipples swell to several times their normal size, the large puffy nubs aching to be touched. And he felt the flesh around them expand rapidly, until what he had on his chest was undeniable. These were boobs. Much as he always assumed this couldn't happen, it was happening and he could do nothing about it- his body was changing sex.
He hastily dropped his jeans and underwear, and watched helplessly- and kind of arousedly- as his body finished transforming. His leg muscles subtly changed shape as his arm muscles had, and his feet turned more petite while from his waist downwards much of his body hair thinned out. He felt his butt swell and saw his hips widen to accommodate it.
With almost all of him now her, the sharp tingling accumulated around the only part of his male body still not changed- and to his surprise, instead of hurting it felt pleasurable beyond anything he could've imagined. As his penis started to shrink away the nerve endings bound together tighter and tighter, and when his testicles moved up inside him something clicked in his-or rather, her- mind. She was not a man anymore, but a woman, and this was not his body mutating but her body taking shape. She even adopted a different name in her head- Sylvia. As a result, it was with a sort of relief that she felt his penis finish receding into a tiny clitoris and his scrotum sink inwards, unseal itself and form her vagina at its centre.
Sylvia looked at herself in amazement with her eyes glazed over for a while, and slowly regained her focus. Wow, I really have turned into a girl, she thought to herself, wondering what to do about it...