You might be slightly allergic to lactose which is why they suggested an isolate. Plus you are switching to Waxy Maize as your post workout carb, right? Sometime the old Maltodextrin and Dextrose carbs made me gassy.
What I do is put in a shaker Whey Isolate (40 g.) and Waxy Maize (60 g.) and keep in the locker. After my workout, go to the fountain and fill er up. Drink it before I even get to the front door. Then I eat about an hour later.
Good luck bro, in my 18 years of this, I found I am always tweaking and adjusting my supps and diet. After all these years of eating all this fucking food, my intestines must be a wreck. Which is an old guy like me is doing more enzymes, probiotics and greens.