Lean body mass gain is more than just contractile protein gain (and its accompanying water) because muscle glycogen (stored form of glucose) and its water, sarcoplasm, etc. fall under LBM poundage. Like, even after just one carb load, some people gain 5-10 pounds without skinfolds increasing which would've been a a strong sign of body fat gain. The water got soaked up into lean tissue.

The thing about whey giving you digestive problems but not milk is slightly weird. Most digestive issues are due to lactose. And per gram of protein, milk (which is ~20% whey) in most cases gives you much more lactose than whey powders. Must've been the other stuff in the various whey powders that was the cause.