Sharp pain. What could this be?!

So, I'm 15 years old (and not sexually active, if that means anything for the answer) and every once in a while I get the really sharp stabbing pain in the lower right corner of my tummy. Its just above my hipbone, and over... about 3 inches down, and 4 inches over from my bellybutton. It's really painful, and it gets worse when i lie on my stomach, or stand and twist back and to the left. What could this be? And I don't want any answers like "Don't twist back and to the left." Serious answers only please...