If you are truly experiencing stomach problems with ONLY dotFIT(dF) powders, you are in an extreme minority. And you can only gain weight when you consume more calories than you burn so protein by itself does not cause weight gain. The proteins in the dotFIT products come from all the same sources that all other dairy proteins (whey, casein, etc) originate from. Additionally, there are only trace amounts of lactose in the dF powders, which is not enough to bother anyone that is lactose intolerant unless 100% allergic/intolerant, which is rare. See related question here http://www.dotfit.com/content-35755.html
Some people are sensitive to any protein powder because it digests quickly, which can give that uncomfortable feeling of fullness after consumption. However, that feeling subsides quickly. Also, most "protein" powders/drinks contain carbohydrates of differing types and specific ones (especially fiber) may cause a bloating feeling, especially if consumed quickly, in certain unique individuals.
Other potential reasons:
• Soy allergy: there are small amounts of soy protein in many of our products
• Sugar alcohols: there are small amounts of sugar alcohols in our bars
• Consuming the shake too quickly or with other components that may be lead to stomach upset