Every so often i get a this sharp pain on my upper left side, below my breast under my rib cage, and it lasts for a while, what might it be?

Upper chest pain: A sharp "knife like" pain, usually activated after a deep breath which disappears after less than a minute can be due to the lung not having been fully inflated and therefore stuck to itself partially collapsed. It can be the result of a "sticky" mucus on the surface of the lung. If this continues, see your health care provider for testing.