treated water and stomach aches

You know, the water cooler water that you get in those giant jugs, often Culligan brand. That’s what I’m talking about.

Anyway, for some reason, that water gives me bad stomach aches. I’m guessing there’s some sort of chemical used in the purification process that I’m reacting to. And it’s just the water from those jugs. I can drink water from a Brita filter device (charcoal filter) just fine.

Does anyone have any idea what in the water could be irritating me? I tried Googling for specific details on the chemicals in the water or the process used but I’m not coming up with much.

As an aside, this tells me that bottled water (Dasani, Aquafina, etc.) doesn’t go through the same process, because those don’t make my stomach upset either. And people always look at me funny when I go for tap water instead of the water cooler.