When Mark was a smart kid, at least academically, he even skipped a grade. So when he had to watch some health videos about puberty with his classmates he wasn't quite there yet. At the end of class when he was talking to some friends about the video he ended up giggling and finding the idea of a period funny... Although his friends didn't quite find it as funny, and thought that Mark was being a little too loud. In truth he very much was too loud. There was a girl in his year named Sam who was kind of creepy, and believed herself a witch of sorts and was looking through a book of some curses... Although didn't use any of them as she hadn't found a good target yet, or hadn't.
Sam overhearing Mark's laughing at periods got a little mad at him, just having her first period and it was a bit of a rough one, so she went towards him and said "If you find periods so funny, then i'll i'll... Curse you so you can have one each month!"
Mark just laughed and said "Oh god, do you really believe in curses? What are you just going to try to make me horny each month or something?" Sam's face went red with both frustration, embarrassment, and anger.
One of his friends said "You know whether or not curses or real you have been acting a bit of a jerk. Just cool down a bit." to which the rest of his friends agreed and left Mark to himself for awhile.
To which all he thought was "Meh, humor is supposed to be somewhat offensive. It's only going to make sense that someone is going to be offended." and continued with his day a little more to himself. That evening however Sam was making good of her threat and found the right elements needed for a curse. It wasn't exactly making him a girl only when he would have a period, but it was close enough for her. She did her ritual and then... Well Sam had a lot of explaining to do to her parents why her room was filled with smoke. Basically she'd be grounded for awhile.
Meanwhile at his home Mark was watching some videos online when he felt odd. A sort of creepy sensation on his back, and then the introduction of the curse came with a little extra "kick". Watching the video he wasn't quite paying attention to the change itself which he didn't really feel that much. His dick quietly slunk into his body as it changed into a vagina, his features softened a little as his voice would get a little higher pitch. His chest pushed out ever so slightly as well, but at this point becoming a girl would become her second most pressing issue as she was getting hit hard with some of the period symptoms. Cramps, a bad headache, feeling kind of drained, etc. At least though she had a general idea that she should get to the toilet which she did. Taking her pants off to sit on the toilet quickly noticed she was indeed a she and in a few moments the stream came out. She kind of groaned as she felt rather sick.
Mark's mother going by the bathroom asked "Mark is that you? Are you ok?"
She wasn't quite expecting a more feminine voice saying "I think I need to see a doctor..."
Mark's mother said "Do you need help can I come in?" Mark grunted yes so she came in and noted that there was a girl who resembled her son on the toilet and wearing his clothes.
The girl looking a little pale said "I think I actually got cursed..." The two talked for awhile and her mother helped her new daughter through her first period, and getting a doctor's appointment for the next day. Which on the bright side meant that Mark got the day off of school, with of course the downside being that she was kind of having a big medical emergency right now. Course the doctor's day was made when the girl that they were examining suddenly grew a cock mid examination... which kind of gave them enough proof of the gender alteration thing. Hearing the story and with the boy not being unhealthy told them to keep track of him in about a month and see if he turns back into a girl. Then all they could say about the girl part was that the girl mark seemed to be roughly as healthy as he was. Then thankfully he ended up being able to get back to school a bit after lunch.
His friends ended up asking things along the lines of "Did Sam do anything to you?" and "What happened?"
Mark wanted to keep his little period thing a secret said "I was feeling a little unwell, and my mom got a little scared. It's nothing." He did end up going to Sam however and apologizing then asked "So was this a one time thing?"
Sam shook her head and said "This'll stay indefinitely."
"Can you stop it?"
"Nope, there wasn't any good ways to stop a curse. Kind of a reason why you shouldn't piss off someone with curses and maybe will do something regrettable on a whim." although with the smirk on her face she sure didn't regret actually cursing someone... Maybe even worth the whole grounded thing.
Mark for sure wasn't finding this whole period thing funny anymore.