Does this happen to you: You are some unspecified, incoherent species of annoyed with the world? Where service workers are likely fortunate not to have to deal with you? Where, if your mood were to physically manifest around you, you'd look something like Pig Pen from Peanuts, lost in a cloud of grunge and blackness? And the incoherence of the source of the irritation (and thus your inability to address same and get rid of it) increases your baseline level of irritation?
It could be the sudden cold snap or the time change. Or it could be the fact that it takes me eons to fall asleep these days (random coworker greeting from this morning: "Jeez, did you have your late class last night? Your eyes are all red"). Bah.