What Causes Pain Around Lower Rib?

I have a sharp stabbing pain that runs along the left side of my body around my lower rib area. It is worst when I am standing or moving and take deep breaths. When I m sitting it is fine and I can breath in and out with no pain. Yesterday the doctor prescribed for me to drink a bottle of magnesium to help the ball movement. That doesn t seem to be helping me at all. Not sure if it s my spleen or what.