Oatmeal should not irritate your stomach, unless you have an underlying condition that would cause inflammation in your digestive system. Oatmeal is a bland food that is commonly used to treat an upset stomach. If you develop an upset stomach after eating oatmeal, discontinue eating it and call your doctor for an evaluation. Common conditions that would cause irritation to your stomach from eating oatmeal include celiac disease and food allergies. Oatmeal is a high-fiber food, which also could cause temporary digestive side effects.
Celiac disease is a chronic digestive condition that causes your immune system to harm the lining of your digestive system when you eat gluten. Gluten is a protein that is found in rye, barley and wheat that is safe for most people to eat. The condition is not fully understood, and the only treatment is to implement a gluten-free diet. While oatmeal does not naturally contain gluten, most oatmeal is processed on shared equipment with gluten-containing grains. Unless the oatmeal is labeled "gluten-free," you should not eat it if you have celiac disease.
It's possible to have an allergy to oatmeal. While oatmeal is not considered a common food allergy, you can have an allergy to any food. If you develop stomach pain, diarrhea, nausea, vomiting, bloating and cramping after eating the food product, contact an allergist for testing. An allergic reaction to oatmeal is caused by a hypersensitivity of the immune system to the proteins in the grain. The immune system attacks the proteins in the oats with antibodies, histamine and other chemicals that cause common allergy symptoms, according to the American Academy of Allergy, Asthma and Immunology. If you cook oatmeal with milk, which is often done in restaurants, you may want to consider being tested for a milk allergy.
Oatmeal is considered a high-fiber food by MedlinePlus.com, containing 4 grams of fiber per cup, cooked with water. MedlinePlus explains that when you increase the amount of fiber in your diet suddenly, you may develop excessive gas, bloating and abdominal pain. If you continue to consume the same amount of fiber daily, these symptoms should subside within a few days. If you develop severe abdominal pain or diarrhea, stop eating oatmeal and call your doctor.
If you develop other symptoms, such as fever, chills, fatigue or vomiting, you may have a more serious medical condition, such as gastritis or food poisoning. Some people who have irritable bowel syndrome may experience digestive symptoms from eating oatmeal.