Hey AMC,
I believe Emeric is correct, but here's a little more info.
Btw, 500 grams of protein is very harsh long term on your system and cannot be utilized by your body. Just ain't gonna happen.
Hope this helps:
This may be caused by a number of different issues, but our initial thoughts would be that you have developed an intolerance to lactose or dairy due to the large amount of material that you are consuming daily. If you are only drinking a small amount of milk, it may not show noticeable effects, but when slamming down hundreds of grams of protein, the overall lactose content may now be affecting you. If it is a new sensitivity to lactose or dairy, you would experience the same issues when switching brands. Another possibility is that you have developed an allergy to soy, which is used in minute amounts in the instantization process for nearly all whey proteins on the market. This might be worth looking in to as well. Possible alternatives would be to use a vegetable based protein powder like the Gemma Pea Protein or Rice Protein,
Hemp or the Tri-Blend. It's a myth that all your protein has to come from animal sources. Or try a material like 100% Egg White Protein or Beef Protein Isolate to see if this resolves the issues. If you have been using Whey Protein Concentrate, I would recommend using Whey Protein Isolate in its place as well, which has nearly all of the lactose removed and is much easier to digest.
MT