Lower back pain/spasms?

I'm 17, and healthy. I do tae kwon do 3 days a week, and work weekends in retail. Recently, I felt a sharp pain in my lower back, right in the center about 3 inches above my waistline. It hurts sharply and suddenly at various times when sitting down, twisting my waist a certain way, and walking. I can't recall of doing anything extreme to hurt my back or anything; any diagnosis or advice? Thanks