Hi Michel.
I have read many articles and studies about this topic from the day doctor told me about my diagnose. Many studies say that a mature human CAN NOT digest lactose, because the lactase just disappears from their body when they get mature. Only small children can digest the lactose...
However my whole family eats dairy products and does fine. It may vary from person to person I think. If you feel that way, definitely go see a doctor.
There are 2 common ways of discovering whether you miss lactase or dont.
The first one is by blood tests - however, my blood tests were fine (even the repeated ones). The second one is biopsy of your duodenum. For me only the biopsy results discovered my illness. Dont be scared of the biopsy - its not lets say comfortable but its definitely not that bad and its nothing you should be afraid of
Or you can try not eating dairy products at all. This may be difficult because for example dairy is found in milk chocolate etc. which means you will have to really look on what you eat.
If you try this lacto-free diet, try to withstand it for at least a month or two. If you suffer from diarhea or something the results may show up even after a few days but long-term symptoms will like acne etc. will need more time to get cured.
Hope you get well