What rarely happens - a colonics tech told me about this - is that one of the tendons holding the common bile duct and small intestine in place will get kind of kinked up and wrapped around, causing pressure/pain that is often mistaken for a stone. Fiber plus massage, or colonic plus massage, is all you have to do with this issue. Since sitting means being hunched up in one position for a long time, it could be putting extra pressure on that tissue. And a castor oil pack would probably be a good idea too - you can find out about how to do that here on CureZone.
So ... try heat/massage and then if the pain subsides resume the P&B shakes as the fiber type exercise will massage it too. :) Let us know what happens! :)