Lower Back Pain...................?

Im almost 18 years old and 2 weeks ago as i was picking something up (a pencil nothing heavy) my lower back began to hurt. it kept hurting for about 2 hours but then the pain stopped. i thought it was something that happened and that its gone. however, for the past 2 weeks the pain comes and goes escpciially when i bend down. it has not effected the way i walk, or sleep, and i can still excersise(i mainly run, and swim, and i dont always stretch so can that be the problem?). so is it anything serious, should i check a doctor, what could possibly be wrong?