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I would have walked […], but I felt funny when I got outside. Sort of dizzy. (24.2)
At first, we can write off Holden's not feeling well as the result of the alcohol and the lack of sleep.
"I had this terrific headache all of a sudden." (24.21)
Since he keeps mentioning this headache business, we're sort of interested, but again, it doesn't seem like anything serious at the moment.
"I still had that headache. It was even worse. And I think I was more depressed than I ever was in my whole life."
"So I went in this very cheap-looking restaurant and had doughnuts and coffee. Only, I didn't eat the doughnuts. I couldn't swallow them too well. The thing is, if you get very depressed about something, it's hard as hell to swallow." (25.3; 25.7)
Holden explicitly connected mental fatigue with physical sickness. His breakdown, then, is most likely a combination of the two ailments.