Amy Schumer was 10 years old the first time her period pain knocked her flat, and even then she didn’t think to ask why. For years, she’d watched her mother crumpled on the bathroom floor, suffering from the same debilitating cramps, and assumed it was just what being a woman felt like.Amy Schumer was 10 years old the first time her period pain knocked her flat, and even then she didn’t think to ask why. For years, she’d watched her mother crump…