Why does my poop become whitish a bit loose and gassy with bloating and occasionaly stomach pain when i eat 900ml yoghurt in day for two days?

See your doctor: White stool is usually a sign of a problem in the head of the pancreas or a blockage of the bile duct. It is unlikely from yogurt. The only explanation for the yogurt connection would be a very rapid transit time due to lactose or food intolerance. This could be something simple or very dangerous. You should see a doctor and get checked.