Protein Shakes Causing Loose BM's

You may be lactose intolerant, and whey protein is full of it.  Try some egg-white protein instead.  Jay Robb brand isn't bad.

Edited to add - artificial sweeteners sometimes cause extreme effects of the same,too.

Lactose indolence can be part of the cause and that usually gets way worse after RNY.  There is an explanation for that, but can't remember why.

You are putting in liquid instead of solid, so putting out liquid instead of solid.  Add a fiber supplement and that should firm things up again.