Digestive problems after meals can have many causes. Food allergies occur when the immune system mistakenly reacts to normally harmless foods and can sometimes trigger severe symptoms, including anaphylaxis. Food intolerances, by contrast, are usually caused by the body's inability to properly process certain food components. Common examples include lactose intolerance and fructose malabsorption. Celiac disease is different again: it's an autoimmune condition in which gluten triggers damage to the small intestine. What symptoms should you look out for, how are these conditions diagnosed, and when can changes in diet provide relief? Understanding the difference is the first step toward effective treatment.