I havent been working for a while now because Im a full time student.  Last week I work for 3 consecutive days stading, moving and walking around for 7-8 straight hours.  Then I slowly realize that my lower back hurts so bad, and now Im in school it still hurts but not too much as it used to.  So why does my lower back hurts?
Any kind of stress in the body can lead to injury, I think there must be a lot of pressure in back muscle, this is normal of exception, you can take muscle relaxant and things will be back to normal, even if you work for long hours, try to do somebody stretch outs, it helps to relax the muscle.