It was a calf cramp, of course. Please, no quack doctoring; I know what causes them and how to deal with it. Comments are turned off.
I knew I was a little dehydrated -- I worked up at the North Campus yesterday, where it is cold and dry; even the enclosed garage where I parked was at 20°F. My former office and workshop was somewhere between 50° and 62°, depending on which end you picked. The actual equipment rooms were between 65° and 70°, but very dry.
Because of the extreme cold, I wore heavy boots, though I changed to sneakers for most of the day. Trudged through the supermarket in the boots when I stopped on the way home.
So, dehydrated, muscle strain -- the surprise is that the cramp waited until 3:30 this morning to hit. I'm still limping around. The aftermath is probably going to linger.