The answer to the crossword puzzle for the term 'Cause of Much Bellyaching?' is 'ACID'. ACID reflux, also known as gastroesophageal reflux disease (GERD), is a common condition that can cause much discomfort and pain. It occurs when stomach ACID flows back up into the esophagus, causing burning sensations in the chest and throat. Symptoms of ACID reflux include heartburn, chest pain, difficulty swallowing, and a sour taste in the mouth. Treatment options for ACID reflux include lifestyle changes, medications, and surgery.