Random pain in my spine?

months i can be fine, but outa know where, ill be walking around or sitting on the couch and ill get this sharp pain in my lower back, sometimes it hurts bad sometimes it hurts severly, and the pain will last for a copuple hours to about a day and after that im fine. but the pain feels like somone it taking a knife and sticking it ito my spine and ripping my spinal cord. im only 19 and i dont think it has to do with age. im a little on the thick side but i know it dosent have to do with weight becase im verry active, and i dont remember or recall getting injured? anyone have the same problem or have any ideas? i try taking mortin and other medication and it dosent help, and i would go to the doctors but i dont feel like het telling me something i already knows, becase i already did and she said it could be my weight..... i dont think it is becase i may be on the thick side like i said but im a verry active person and its not like i sit on my *** all day