I can tell you with 99% certainty that your GI issues and the diarrhea is from your protein supplement. Read the ingredient list and I bet you it will list 'whey protein concentrate'.
There are two main types of whey protein available: a whey protein isolate and a whey protein concentrate. The problem with a whey protein concentrate is that it will contain a few grams of lactose (the milk sugar naturally occurring in dairy products) per serving. The majority of adult humans have a lactose sensitivity which can cause GI issues like gas, bloating and diarrhea. A whey protein isolate on the other hand is essentially pure protein and will have only trace amounts of lactose.
When purchasing a whey protein isolate you want to see nothing in the ingredients list besides whey protein isolate, natural flavours, a natural calorie free sweetener like stevia and maybe some salt, cocoa powder and soy lecithin.
I personally use the whey protein powder isolation from n2bm.com. A 30g scoop has 26g protein, 1g carbs and 0g fat.