I don't want to list everything that happened, but about four or five really irritating situations were stressing me out throughout the first part of the day, but then things boiled over to me when (a) my direct boss (not the one who sent me home) started joking loudly two feet from me while I was trying to make phone calls he'd just told me to make...and (b) one of my co-workers kept asking me to guess what song he was humming, and after I couldn't guess and started asking him to stop humming, he waited a little bit and then got behind me and hummed loudly and directly into my ear.
Thing was, he took me by such surprise that I turned around with my arm raised.
I didn't hit anybody, but my real boss saw what happened, and I quickly walked away from my desk, really flustered, and tried to calm myself down outside the building.
When I got back in, he told me that I should go home for the day.