Some months ago walking got suddenly painful in the lower back. Walking down a stair was only possible at half the normal speed. I am fourty-something and this made me feel very old. I did more of my usual back-strengthening exercises, but it got worse. I thought surely something is broken. When I went to the doctor she told me that I just neglected stretching, mostly the hip flexor. It went away after doing that. Apparently very common when you sit a lot, and when you do lots of running. (And I did try more running to make it go away, without stretching afterwards, lol.)