It's Chapter 17 of Are You There, God? It's Margaret and Margaret has been invited to spend the day in New York with her friend Nancy and her family. Moose Freed will also be there, and that's an added bonus because Margaret has such a huge crush on him.
When they're in the bathroom stalls, Margaret hears Nancy crying and wonders what's wrong. It turns out that Nancy's having her period, her first one, at that. But Nancy had previously told Margaret and the other girls in the Pre-Teen Sensations that she'd already started menstruating. This was because Nancy was so obsessed with appearing sexually mature in front of everyone. Margaret's upset with Nancy for lying to her; she feels almost betrayed by her actions. Nevertheless, she agrees not to tell anyone else about it. Nancy may have betrayed Margaret by lying to her about her periods, but Margaret's not about to betray a confidence.