Several factors can contribute to stomach pain after eating cereal and milk, such as overeating, but if it happens consistently, a digestive condition could be to blame. For example, the inability to digest a sugar found in milk -- a condition known as lactose intolerance -- is a common cause of stomach pain. It's also possible that something in your cereal, such as fiber, is the source of the problem. It can be difficult to pinpoint the source of the problem on your own, so if it occurs consistently consult a qualified health practitioner to help determine the underlying cause.
Cow's milk contains a sugar called lactose. Breaking it down in the digestive system requires the presence of an enzyme in the small intestine called lactase, which some people don't have. If the lactase enzyme is missing, the lactose has to be broken down by bacteria in the large intestine, which can cause gas and stomach pain -- as well as bloating or diarrhea -- in some people. Not everyone will have the same symptoms, and stomach pain may be the only symptom of lactose intolerance. Between 30 million and 50 million people in the U.S. are lactose-intolerant, according to The Ohio State University Wexner Medical Center.
The proteins in cow's milk are a common allergen, particularly for young children and infants, according to the Food Allergy & Research Education website. Many children outgrow it, but it can occur through adulthood. Unlike lactose intolerance, an allergy to milk can be life-threatening. The symptoms of a milk allergy are similar to those of lactose intolerance but may be accompanied by other symptoms typical of allergies including itchy skin, numbness around the mouth, hives or a runny nose. A severe allergy can cause the body to go into shock, which can be fatal if untreated. Stomach pain may be the only symptom of a milk allergy for some people.
Your cereal could be causing stomach pain as well. Fiber, a common ingredient in cereals, can cause gas and stomach pains, particularly if you are not accustomed to eating it on a regular basis. One of the cereal's other ingredients could potentially be causing problems, too, either in the form of a food intolerance -- similar to lactose intolerance -- or an allergy. Wheat, for example, is found in many cereals and is a common cause of allergies. Gluten -- a protein found in wheat, barley and rye -- can also cause stomach pain in some people.
Stomach pain that occurs once or twice after eating cereal and milk could be due to overeating or a very mild case of food poisoning. An isolated incident like that is not generally cause for concern. If the problem occurs consistently, however, it's best to get checked by a doctor, even if the symptoms are mild. Your doctor can run tests to possibly determine if you are lactose-intolerant or have a more serious allergy to milk, or an allergy to one of the ingredients commonly found in cereal. In some cases, a previously undiagnosed digestive problem, such as irritable bowel syndrome, could be to blame. Dairy products and fiber are common irritants of irritable bowel syndrome.