what causes an enlarge bile duct

I had pain on my right side going into my back. Sometimes it was a continuous pain while other times it felt like a knife being stuck into me. This has been going on for months. They did a liver test, kidney for stones and pancreas which all came back normal.  Finally they did an ultra sound which showed my bile duct was enlarged.  What would cause that?

[CalGal]

It's typically attributed to some form of blockage of the common bile duct due to stones. However, in some cases, such as after the gallbladder is removed, enlargement can occur but there is no explanation for why the enlargement is found.