Most probably it has to do with all the added ingredients, such as flavorings and sweetners. Also, it may contain lactose, and your stomach could be sensitive for it. Therefore, you may as well try to make a switch towards a different brand, and try Whey Isolate, since it does not have the lactose in it. Isolation is one awesome product. Finally, you are taking quite a lot of protein, so you should consider lowering the intake.