Question

Why does Margaret become upset with Nancy?

Accepted Answer

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.

Answer

Margaret became upset with Nancy because she had lied to her about getting her periods. In chapter 17, Margaret gets an invitation to spend a day with Nancy’s family in New York. Margaret’s crush, Moose, is also there with them. At the restaurant, Margaret enjoys being next to Moose and bumping elbows with him.
It is while Margaret and Nancy are using the bathroom stalls at the restaurant that Nancy begins to cry. Worried, Margaret goes to fetch her mother who discloses that Nancy was experiencing her first period. This comes as a shocker to Margaret who had earlier received a postcard from Nancy indicating that she already got period. The postcard from Nancy read as follows;
 “I got it!!!” (15:79)
Margaret is disappointed and upset because Nancy had lied to her. Nancy pleads with her not to tell anyone about the incident. Eventually, Margaret agrees but still feels upset and betrayed by Nancy as evident in the statement below;
“Are you there God? It’s me, Margaret. Nancy Wheeler is a big fake. She makes up stories! I’ll never be able to trust her again: I will wait to find out from you if I am normal or not.” (17:42)
As a result, the friendship between the two girls grew weak.  From that day on, Margaret did not have to worry much about what Nancy thought or said. Instead, she turned all her troubles to God.  
FURTHER READING
https://www.enotes.com/topics/you-there-god-its-me-margaret

Are You There, God? It's Me Margaret

Why does Margaret become upset with Nancy?

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