For Dean to run an errand for her

“Run to the store and pick up some tampons for me,” she tells him half expecting him flatly refuse “I’m all out and I just don’t have the energy to go myself.”

“Aww, man I can’t believe you want me to do that” he moans “You have no idea how embarrassing it is buying those things for my sister.”

“Ha,” Karen answers with a tired laugh “if you think it’s a pain buying them you should try actually having a period. Having to use the damn things is a lot worse I can tell you.”

Embarrassed and still grumbling Dean pulls on his jacket and heads for the store, it’s not until he has ridden halfway there that it occurs to him that he could have just refused to go. He thinks about turning his bike around and going home but somehow it just doesn’t seem worth the effort and before he knows it he is at the store. He makes his purchase without meeting the clerk's eye then hops back on his bike to head home.

It’s not until he is riding home that he starts to feel weird at first he thinks there is something wrong with his saddle. He keeps stopping to adjust it but no matter how he shifts it still feels uncomfortable. Like it’s pressing against him more than it should, almost like it’s giving him a wedgie but in the front rather than the back. Eventually, he gives up on fixing it and decides to just get home as fast as he can. As he rides the feeling gets stronger but at the same time it starts to feel kind of nice, every bump in the road makes him gasp and squirm until without even thinking about it he is deliberately pressing himself against the saddle trying to find the most sensitive spot to ride on. By the time he gets home, he is red-faced and panting and not just from the effort of ridding. As he dismounts he realises something else, he really has to pee!

He rushes straight to the bathroom, ignoring his sisters greeting. Still holding his purchase he locks the door behind himself and unzips his fly but when he reaches for his dick he can’t seem to get hold of it. **** to relieve himself he drops his pants then his boxers and is horrified by what he sees. Instead of his usual male equipment, he sees a thick thatch of pubic hair covering a small mound and an unmistakably feminine cleft. Too astonished to even move he can only stare as he loses control of his bladder and piss sprays out of him running down his leg and pooling on the bathroom floor.

"Fuuuck!" He squeaks hoarsely "That's a pussy! Why the fuck do I have a pussy?"

He grabs himself feeling it's soft outer lips part under his fingers and touching the sensitive folds within. He curses and yanks his hand away thinking that he must be tripping on something but he knows he is not. From the smell of urine to the alien sensations coming from between his legs it all feels too real to be a dream or hallucination. Just when he thinks things can't get any worse he starts to feel sick, his head aches and his stomach feels bloated. he puts it down to shock but then the stomachache becomes a knot of pain that doubles him over. Again painful cramps hit him and he feels warm wetness trickling down his thigh, for a moment he thinks that he has pissed himself again but when he looks down he sees blood leaking from his new hole and it dawns on him that he is having a period.

This is just too much for him and he faints losing his grip on consciousness and crashing to the floor...