What Causes Mild Dull Pain Around Gall Bladder Area?

I have had a mild dull pain around the gall bladder area. But nothing else. The pain is more nticeable when i exercise, swim, etc... Of course l thought about pancreatic cancer!! The pain comes and goes and i hardly feel it if l put my hand on the area while doing sit ups for example. My doctor sent me for an ultra sound of the abdomen since the pain is there on and off, but as l said, very mild. What can it be?