Symptoms

stomach aches,  flatulence, feeling of fullness and lose motions after the consumption of milk

If the enzyme, lactase, is no longer present in adequate quantities in the body (Lactase deficiency), the lactose reaches the lower section of the intestine (large intestine) in the unconverted form and is then fermented there by the intestinal bacteria with the formation of gas. Moreover, the lactose can cause increased water flow into the large intestine.
This can lead to complaints such as, for example, stomach aches,  flatulence, feeling of fullness and lose motions after the consumption of milk products and is then referred to as lactose intolerance.

On account of the similarity of the complaints lactose intolerance is often confused with an irritable colon.