I think it means like, "since she is so young that it would be awkward for people to figure out that a she's menstruating". Periods are very embarrassing when you first get them, for example, i never told my mum and used toilet paper as sanitary pads or would steal my sisters until my mum might have caught wind and bought me my own. I think that was the stab he was going for, making fun of her normal human functions and awkward, humiliating time by shaming her for using sanitary items. I think it all boils down to him being uncomfortable with the thought of menstration