I would guess that it is not from the insulin but rather from the high osmalalic properties of your post-workout meal. Both glucose and whey protein (or fast absorbing protein at that) have hypertonic characteristics and will draw water into the intestines. Therefore, to decrease this effect you should:
1. Increase water intake to increase both solubility and to decrease the concentration of the meal/solution.
2. Take in half the amounts right after, then wait....oh, maybe a 1/2-hour and then drink the rest.
3. Make sure your protein is of higher quality and has better solubility properties. Whey isolate might be ideal.
4. Ginger root may be of benefit with the digestion and absorption of your protein.
MR. BMJ