Why does severity of pain from gallstones increase during a meal?

Simple explanation: The gallbladder is where the body stores bile, which is necessary to digest fats. When you eat, as the food leaves the stomach in it's partially digested form, known as "chyme" the gallbladder begins contracting to release bile. Then there are two possible sources of pain. First, contraction of the gallbladder tissue against the stones, and second a stone blocking the bile duct. Hope this helps!

GB contracting: to excrete bile, to help digest fatty foods, against outflow blockage ( stones )