Looking at the Muscle Pharm Ingredients, this contains a mix of WPC, WPI, HWI, Micellar Casein and egg white.
It's quick to figure out that the fix to your upset belly has come because you have drastically reduced the amount of lactose you now take in by at least half.
It's still present though with WPC and Micellar Casein.
Eliminating a far greater percentage of the lactose would have been a wiser choice, in my opinion.
A pure WPI (from where ever you choose to purchase from) will have far less than the brand you mention.
At the same time, you have also marginally increased your costs in protein.
When looking at a popular discount supplement website, Muscle Pharm sell their blend for $52.90 for 910 grams.
Our pure WPI for example is $36 for 1 kg.