Acidity and heartburn occur when the lower esophageal sphincter—the muscular valve between your stomach and esophagus—weakens or relaxes inappropriately, allowing harsh stomach acid to splash back upward. This reflux is most commonly triggered by eating habits like consuming large or high-fat meals, lying down too soon after eating, or drinking caffeine, alcohol, and carbonated beverages. Spicy and acidic foods like citrus and tomatoes can further irritate the stomach lining, while physical factors like pregnancy, excess abdominal weight, chronic stress, smoking, or certain pain medications increase pressure on the stomach and compromise the valve’s function.