A lot of things
Pain in this area can be caused by many things.
You will need to see your doctor. He/she will take a good history of your pain, ie its location, is it constant or intermittent, the quality of the pain (i.E sharp, dull, aching, burning etc) whether it is affected by change of position or not, whether it has any relationship to eating or not and many other questions. Then a physical exam and tests.