The ES makes it easier for the stones to move through the ducts. I beleive it's the stones moving through the ducts which is causing the pain. It should start to subside after some more liver flushing. I've found magnesium citrate a better more bio friendly alternative to epsom which can be harsh on the kidneys and intestines.
Chanca Peidra also called stone breaker should also help make it easier to pass the stones.