What Causes A Dull Ache Below My Ribs?

I have had a dull ache on my right side just below my last rib that comes and goes for about two months, it is worst when standing and feels the best when lying down. I know it is not a muscle pain. I have no other symptoms. No loss of appetite, no urine, or stool change, no nausea, now weight loss or gain,no pain anywhere else.