Did you drink? You know ... alcohol? I know you don't drink often, if at all, except for that one time... Hehehe.
ps: You don't have to get drunk to have a hangover from drinking. I drank very little on New Year's, but still had a headache the next day. Rather disconcerting.
I had a minuscule amount of champagne at midnight on New Year's, but it's a no-sleep no-caffeine headache. After a night of decent comatosity I finally beat it for today.