After I made the decision, came home instead of catching the train, sent the e-mail, I took a nap for a couple of hours. Of course, now I'm feeling fine, and guilty that I'm not at work.
I suspect that what I was feeling was some combination of the allergy/sinus thing that has been plaguing me for a few weeks and yesterday's flu shot. robot_grrl was knocked back a bit by her flu shot as well, so this year's blend seems to trigger a more active response in the immune system than the ones for the last few years.
Hopefully, in a few weeks I'll know how the H1N1 vaccine hits us.