Why do I get headaches when I drink water?

When I drink certain things, I often get headaches. It mostly happens when I drink plain (tap) water, bottled water, low calorie drinks (like G2 or propel), and things sweetened with splenda. However, I can drink diet coke, and when I do drink water, it HAS to be iced or it has to have something added to it like lime juice, or those 5 calorie iced tea singles. Why?