I've been suffering from a bad back for nearly two weeks now - a sciatic nerve thing that I get occasionally, but which usually clears up in a day or two.
I never have back problems cycling or at the gym, I guess because I always do stretching and warming up - when my back goes, it's always doing something innocuous like standing up out of a chair, or picking up a bag, or something.
Fortunately, with regular stretching exercises, it's gradually getting better. And being on a bike is helping, so I've managed to do a couple of rides this week. But lingering back pain is very annoying.