Upper-left abdominal pain

Yesterday, after being awake for a few hours i noticed a dull pain in the upper left side of my abdomen, closer to the back.  The pain has gradually gotten worse, but not unbearable.  I can always feel it when I breathe, but it flares up a little when I take a deep breath and then goes away almost immediately.  Today when I woke up, I thought the pain was gone, but as soon as I sat up, it started again.  If stand or lay down for a few minutes, the pain subsides (even if I take a deep breath).

Does anyone have suggestions as to what might be going on?

thanks!