Oh wow this sort of thing really hits home. Very depressing and I can feel her struggle. Endometriosis and other similar conditions are terrible, for me that it feels as though someone is grabbing your insides, twisting them or hitting them for hours. Plus a roulette of other random symptoms. Even normal periods can still sap your physical and mental well-being significantly. I also don't know that I've seen this subject noted upon in manga, only Berserk comes to mind. Human females got a very short stick with this path of evolution. Even if it wasn't just about her period, it still touches upon a dismal subject of absolutely putting in your all but being ultimately being limited by biology beyond your control.