It takes a certain kind of enzyme to digest proteins (milk), and a completely different enzyme to digest citrus. When taking these two together, the body gets confused and the digestion process is slowed, leaving milk curdled in the stomach. This causes nausea, bloating, and constipation. Besides, when taken with citrus fruits, milk coagulates as these fruits can change the pH level in our stomach, making digestion process even harder to proceed.