Up until the last few days plain water has been going down with no problem at all. But, over the last few days it has been upsetting my stomach. Sometimes I'm even having some cramping. If I put some flavor in it, though, I don't have any problem. It sounds weird doesn't it? Does anyone else have this problem?
try playing with the temperature of the water. some folks need it cold, others cool, still others warm, and some can't drink anything but hot beverages...
and be sure that you're SIPPING. it's possible that you're SIPPING the water when you have some flavor in it, but gulping it when it's plain.