Skiing and biking all winter. No serious accidents this year, no minor falls recently. Yesterday, just sitting in my office chair, I had a sudden very sharp pain in my lower back. Hurt to move. Pain seems to be subsiding (on day 2 now), but still struggling a little to walk normally, put on socks. Still hurts.

What kind of injury is sudden, without an obvious instigating event? I just want to know how long it will hurt.