I think I just found out what's been plaguing me for a year and a half, after searching on the internet; it matches the symptoms exactly. It's called precordial catch syndrome, or Texidor's twist. I get a sharp pain under my left ribs, and a sharp pain in the chest when inhaling, usually when laughing or talking without taking a breath. It's like someone is twisting my heart, though it's not considered a cardiac event. Apparently it's prevalent in childhood and adolescence, but can occur in adulthood. It's benign, and they can't really surmise as to what causes it, and they don't treat it. Very interesting.