Jem is described as “moody” after he returns from retrieving his pants.

Jem’s moodiness is actually a sign that he is growing up, and Scout and Jem are growing farther apart.  She did not understand why he had to go get the pants.  It was an adult decision to try to rectify a childish mistake.

Jem knew that Atticus would be disappointed in him for going to the Radley house to bother Boo.  Jem did not want Atticus to know that he went. 

Jem stayed moody and silent for a week. … I tried to climb into Jem's skin and walk around in it: if I had gone alone to the Radley Place at two in the morning, my funeral would have been held the next afternoon. (Ch. 7)

He was more shocked to find his pants mended and folded and waiting for him.  This made him aware of Boo Radley as a person.  He sympathized with him, and the week he spent trying to reconcile what he felt about Boo Radley with Boo’s actions made him moody.

This growing understanding of the world created a canyon between Jem and Scout.  She did not understand his new perspective, and he did not feel ready to share it.  This was the beginning of the strain in the relationship.