I had a feeling... she didn't actually seem all that sick, but something was off about her, huh? It only made sense it was that. And, of course, three months... was probably their first time, at the start of this volume. They haven't highlighted any other times they've done it. It's surprising she hasn't taken note of her lack of periods already- you'd think a few months without a period would be something a girl would notice.
But, well... the author isn't a woman, right? Probably doesn't think of these things. I only know to think of it because I have three older sisters, as well as all the stories I've read by female authors where that was what the women noticed first, or there was noting of periods in general.