If you cant think of any trauma, jolts, falls and the like, its probably a strained muscle. Keep in mind that the disks in the back, especially the low ones, have the most pressure on them in the slightly bent over position. Drink lots of water (and maybe a V8 for potassium) and watch your posture. It should pass. Take care of your back, young man, and it will often take care of itself! Read some of these other posts if you need motivation to care for you spine.