It could be anything. This sounds developmental, FWIW. You've probably considered all the obvious possible explanations (sleep, diet, etc.)

(As a PSA for others, when I hear "sudden personality change/sensitivities" my mind goes to PANS/PANDAS, but it doesn't sound like the sensitivities in this case rise anywhere close to that level; I might think differently if there were specific additional symptoms and/or recent infections)