What could be causing my lower back pain?

It started last monday from out of nowhere. Its on both sides of my lower back near my ***. its not severe or anything but it just aches to the point where i cant stand the feeling of it.i dont know how i could of injured myself other than where i work where i bring in shopping carts alot.. Whenever i wake up it feels better but when i start being active and moving around it comes back.. it feels sort of like a stiffed back. I dont really have trouble walkin, getting out of bed, sitting, and i can still turn my body at certain angles and the pain just stays the same doesnt get better nor worse... Im going to the doctor soon if its not better by wednesday , but i want to know what people think it could be thx for your help