I had Indian food from Madras for the last three meals. I doubt that's the cause, because I eat there a lot. But It's worth mentioning anyway.
It turns out there's a big cold front blowing in. It should be wet and 20 degrees colder by the time it's dark. I wonder if that's what made me sick.
I hate headaches!
EDIT: I forgot to mention that I had taken some Liquid Robitussin Maximum Strength Cough and Cold medicine that was supposed to have expired in 2002. I doubt it was the age, but there might be some ingredient in it that gave me a headache. I guess I'll throw the rest away.