Ooo....

I would like to know your opinion on why, in any genderswap fic, if the genderswap is male to female and menstruation is discussed, the male-to-female character always has killer cramps and is in agony? I've been reading such fic relating to Supernatural, but I know it exists elsewhere as well, and I don't get it. I'm not always in agonizing pain, so why are they?

That's been bothering me.