You know, this series made me think about how few manga (none I've read) bring up the issue of menstruation, as if it doesn't exist. Que fuck, Japan?
I actually read a manga recently, it was none other than nisioisin who brought it up, and with a magical girl character at that! There was several comments about how talking about it is frowned upon in the conversation, so maybe that's why.
Apparently there's a tradition of giving red rice to the girl the day after. That also was brought up in the conversation...