The passage on pages 103 and 104 of The Catcher in the Rye relates to the theme of mental illness, which is prevalent throughout the book. It's interesting to note that the passage begins with Holden admitting that something is wrong with his mental state. The paragraph begins,
But I'm crazy. I swear to God I am.
On the one hand, it's important to recognize that Holden is telling the story in a flashback and is currently receiving some form of treatment for his mental health issues. However, it's clear that in the moment, Holden doesn't recognize the fragility of his mental state. The reader can see this because after admitting that he's "crazy," Holden has what most would describe as a delusion.
About halfway to the bathroom, I start pretending I had a bullet in my guts.
This passage comes immediately after Holden is punched in the stomach by Maurice, a pimp who punches him after Holden refuses to pay him. In his mind, Holden chooses to act as if his injury is worse than it is. Some readers could interpret this as a cry for help, which is common for someone with depression or another mental illness.
Holden then launches into a story in which he describes himself having a gun and seeking vengeance against Maurice, shooting him six times before disposing of the gun. He then blames "the goddam movies" for inspiring his delusion rather than realizing that his distressed mental state may contribute to it.
Also included in his delusional story is a mention of his friend Jane. Part of Holden's delusional story involves calling Jane to "have her come over and bandage up my guts. I pictured her holding a cigarette for me to smoke while I was bleeding and all."
In this instance, calling a friend such as Jane for help could be seen as a rational thing to do. However, in his troubled mental state, Holden continues to choose isolation, even after he's been physically harmed. This adds to the continuing theme of mental illness and the possibility that Holden isn't fully aware of his depression.