I haven't had a headache like that in a long time. The two usual causes for something like that would be a drop in barometric pressure, which I haven't seen any signs of, or dehydration, which is unlikely since a couple weeks ago it was 10-15 degrees hotter and I got by just fine. It's gotta be stress. I've never had something like this happen from pure stress before, but then again I don't think I've ever had to give a presentation to such a large audience before. I practiced my presentation in front of the lab yesterday and it went over well, so I felt like a lot of the pressure was off. So maybe the stress was subconsciously still there (had a similar thing happen with stomach aches in high school) or maybe it's just a response to the sum total of stress from the past few weeks. At least it happened today instead of tomorrow, when I'm actually giving the presentation.
So, um, yeah, no stress, so it should be easy for me to get off of LiveJournal, tweak the presentation a bit, and practice it a few more times. Right...
Better get more comfortable with this if I want to be an academic...