You might find that you can drink milk or eat cottage cheese without discomfort, but experience a slight stomach ache after consuming yogurt for several reasons:

1. Fermentation Process: Yogurt undergoes fermentation, during which lactose is broken down into lactic acid by bacteria. This process can increase the levels of certain byproducts, such as galactose, which might not be well tolerated by some individuals. The synthesis of these compounds can lead to digestive discomfort in some people, even if they can comfortably tolerate other dairy products, like milk and cottage cheese. For more details, refer to the discussion here.

2. Lactose Content: While yogurt generally contains less lactose than milk, it is not completely lactose-free. Some people experience discomfort due to lactose intolerance, which can manifest differently depending on the dairy product. Cottage cheese, particularly if it is well-drained, may contain even lower lactose levels compared to yogurt.

3. Probiotics and Live Cultures: Yogurt is rich in probiotics, which are beneficial bacteria. While they are generally good for gut health, they can cause digestive disturbances in some individuals, especially if consumed in larger quantities or if one has a sensitive stomach. This effect is less commonly associated with milk or cottage cheese, which do not have the same levels of live cultures.

4. Individual Sensitivities: It's also possible that you have a mild sensitivity to an ingredient or additive often found in certain brands of yogurt but not in milk or cottage cheese. This sensitivity could be to the flavorings, sweeteners, or even the textures used in yogurt.

5. Milk Protein Allergies: Even if you don't react to cheese or milk, specific proteins that are present in yogurt (such as casein) might still trigger a reaction for you, especially during digestion.

If yogurt consistently causes discomfort, it might be helpful to monitor the type and amount you consume or consult a healthcare professional for personalized advice. You can find further information about potential allergies and intolerances to yogurt here and lactose intolerance here.

Sources

Reaction to yogurt but no other forms of dairy? : r/FoodAllergiesYogurt Allergy vs. Lactose Intolerance - Verywell HealthLactose Intolerance - Verywell HealthLactose Intolerance: What Parents Need to Know | Lancaster, PAGreek Yogurt Benefits for Lactose Intolerance | U.S. DairyLactose intolerance | Better Health ChannelWhy would Greek yogurt upset my stomach? - Quora

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