My doctor says the pain in my side and stomach are caused by my gallbladder. now i'm wondering what causes the gallbladder to become painful?

Short answer: Is: gallstones cause gallbladder attacks. Usually, a stone, which the gallbladder forms, gets stuck at its neck and causes a blockage against bile emptying. The neck is where the gallbladder narrows down to form the cystic duct, which in turn joins the common duct. When the gallbladder contracts against the obstruction caused by stone in the neck, it causes pain, which is the attack.