What is the cause of a distended gall bladder?

Depends. Your gallbladder will be distended when you are not eating. It fills with bile and then, when you eat, it empties. This is not a problem. It is normal physiology. The problem happens when your gb is obstructed by a stone or inflammation and cannot empty. This can lead to infection and severe pain, fevers, etc. If you have no stones on ultrasound you may need a hida to see how ur gb empties.