I was six miles into my fourth run after resuming training when I felt a twinge in my back. These things tend to sort themselves out. 50 yards later, I was unable to move without severe pain. I managed to get to a park bench and found some gentle stretching relieved most of the discomfort, so I started moving again, slowly. About 1/2 mile from home, it hit again. It took a long time to do that last bit and then I hurt so much I couldn't change clothes (or sit or lay down or lean or kneel...)
Three days later, it occurred to me what had happened. On Saturday, it got to 45 degrees, so I had decided to (unsuccessfully) chip away at an ice ridge at the end of my driveway. It was that weird jarring action that must've tweaked my back, but didn't make itself known until the next day.
It's hard to say what it is. There's a piriformis/gluteus medius problem and a psoas/quadratus lumborum problem, but it seems there might be actual sciatica issues as well. It's getting better, but not fast enough.