Answer:
Basically, Lactose a disaccharides of (glucose and galactose) cannot directly be absorbed through the wall of small intestine to reach the blood stream. Usually they are hydrolyzed by LACTASE enzyme to the constituents monosaccharides which are readily absorbed through the walls of the small intestine.
Therefore, in absence or lack of lactase, there is retention/accumulation of intact / unmetabolized Lactose in the lumen of small intestine, which eventually moved into the COLON.
Intestinal bacterial in the colon breaks down(metabolize) the colon lactose by fermentation, with the by products of CO2, H2, and CH4.
These gaseous by products lead to different abdominal symptoms(bloating, flatulence) experienced by individuals with Lactose tolerance.
Consequently,the increase solute potential of the colon raises the osmotic pressure (since the water potential is very low). These gradients draw water in by Osmosis with the bowel filled with water. leading to swelling