For The Moon

Love is... to make sure your wife is comfortable during her periods, even risking her wrath.

Inuyasha can feel the smell of blood at distance (especially if it is Kagome's), so he must know about menstruation, even if he doesn't know its official name.  
 (I can perfectly figure him out rummaging Kagome's backpack after food and accidentally finding her abs. "Hey, Kagome, what's this?"  "SIT!"  

I made a small research about what women would use in Feudal Japan, but found from little to nothing.. except some notes about a belt to keep the cloth tires on place, which was used during Meiji Restoration.
And that, in Xintoism, mikos (priestesses like Kikyo and Kagome) are not allowed to perform their duties during their periods.

Sorry about the text  - I'm temporarily deprived from Photoshop and Krita's text tools suck, so I had to use Corel Photopaint. I'll fix that later.