The problem started on Sunday morning. My back felt a little stiff, but not unusually so given the weather. The day continued in a relaxed and pleasant way, with coffee, sunshine, and then some moderately satisfying games of Team Fortress 2.
At 7pm, I went to get up off the couch and get a drink, and realized that something was badly wrong. My back was sore and stiff. I used the heat pad for a while, took some aspirin, and had an early night.