I've had a sore back for the past couple of months which has kept me off my bike until recently. I've had this happen before, my back will be sore when I get up in the morning, but after moving around it will feel fine. Eventually it will go away and hopefully not come back, at least for a long time. I'm pretty sure I strained it when I was working on a particular project, standing on top of a ladder reaching up trying to reach something. I didn't feel any pain then but later is when it kicked in.