4 years ago...
Will looked up at the sky innocently. As is often the case for 14-year-olds, he felt weirded out by hormones a lot of the time now, so when he saw the full moon and felt a weird twinge in his body, it didn't strike him that anything was different. He figured his mind must be wandering to horny thoughts, he'd read that was very common at his age.
What finally made him realize something weird was going on was when he noticed the odd sensation that was not only strongest in his crotch, but in his chest. He felt his T-shirt rub against it and shivered in surprise. Pulling it off, he noticed his nipples had perked up, and he gasped as they widened before his eyes. Unfamiliar new desires, just as intense as those he often got from his dick, came from seeing his chest: he started to imagine someone playing with his nipples, rubbing them, pinching them, licking them, sucking them.
As he breathed heavily, flesh started to build up under his nipples. He watched in astonishment as he started to sprout breasts, and was so taken by the oddness of his new mammaries that he barely noticed, much less could describe, how his figure and proportions had adjusted. Everything about him seemed more petite. Prettier. More... there was no avoiding the thought. Female.
He removed his pants and underwear, and despite his wishes for it not to come, the last part of the change was soon upon him. He watched transfixed as his penis shrank and grew more and more sensitive, and his balls were sucked up inside him.
Before long, his scrotum had receded to a labial mound, and formed lips as a vaginal opening emerged where the seal of his scrotum had been, while his penis reached its new, tiny size, having transformed from a thick phallus to a tiny sensitive clitoris.
To his surprise, in the afterglow Will wasn't sure how to feel. On the one hand, he felt a deep sense of castration and emasculation- as if he had lost the person he had been. But on the other, she felt intrigued by her new self. So this is what it was to be a girl.
A moment later, she returned to her senses and her eyes widened. "No!" she protested. "No, no, no... what happened to me? I'm not a girl!"
Frantically pulling her clothes back on, she rushed to her computer and looked up every search term she could think of about gender changing. Nothing even remotely close to what had happened to her.
Scared and depressed by this, she hid in her room until the end of the night, and climbed into bed, unable to sleep with her confusion and fear. To her surprise, just like when she'd been a boy she got horny late at night- but her horniness was fixated on boys. She imagined boys with big thick cocks above her, fucking her wet pussy, fucking her mouth, sucking her boobs, eating her out, even boys fucking other boys. Boys had never felt so different and enticing to her, and she felt so tempted to touch herself, eventually playing a little with her breasts and clit, though not enough to get off.
She was awoken the next morning by rapping on her door and calls of "Wilma!" She was shaken by this, but remembered it was her name as a girl somehow. She dressed in her school uniform, grateful that boys' uniform could be worn by girls, and went to school without a fuss, knowing no one would believe her if she told them the truth. Just to reinforce it, everyone on the buss and in her class was calling her 'Wilma'. Even her textbooks had her female name on them.
In her first class of the day, she found herself looking at the girls he'd had a crush on and still feeling something for most of them, but also spotting boys she now felt attracted to, and her mind idly fantasized for most of the lesson.
As it came to 12 hours after the curse started, she felt deeply odd and asked the teacher to let her go to the bathroom. "OK, but be quick Wilma," she instructed, and Wilma dashed to the boy's bathroom, not even noticing what she'd done.
Hiding in the stall, she watched as her body started to change back. Her curves vanished and her straight, firmer boy's body returned. Her boobs shrank away, to her disappointment as well as relief. And her vagina sealed itself up, testicles moving inside it as it reshaped into a scrotum, and her clitoris expanded until he again had a penis.
Will dressed again and left the stall, a little fearful that everyone would wonder what happened to Wilma. But thankfully, the teacher greeted him with "Nice to see you back, Will." It was, he thought to himself.
Something he found surprising was, as the days passed since the change he realized he had started to miss being Wilma. She had given him a bisexual awakening of sorts as her attraction to boys had carried over once he returned to his old self, and he sometimes felt a phantom limb sensation at his chest whenever he remembered her breasts. He kind of wanted to be her again, but had no idea how that could happen, and no one to talk about any of this with.
At last, 29 days later, the next full moon came...