With all these success stories about people feeling better on various diets, I think we forgot the people who sometimes feel worse. Probably because those people give up and don't stick around. I'm known many people who have adopted paleo, primal, ancestral, low-carb, gluten-free, or whatever diet. And instead of feeling better, they have all kinds of problems, particularly stomach problems.
There are many reasons why this happens, here are a couple I tend to come across: