You may be lactose intolerant if you're mixing your protein with milk or using a whey concentrate which has a lot of lactose. You can take digestive enzymes(lactase I believe) or switch to a more expensive Isolate. I'm lactose intolerant and I use a Milk Isolate and Lactose free milk, no problems now. BTW, you can get a good milk isolate from the Protein factory for $5.99 per lb.