You could be having a reaction to the acids in soda, or even the corn syrup. Have you tried drinking mexican coke to see if it does the same thing?
In any case, it might not be any one ingredient causing it...incidentally, it might just be...soda. Sounds weird but when you put that many chemicals together, then something's gonna happen. No food allergy surprises me anymore, my sister's friend almost bit the bullet after kissing her husband, who drank some beer (because she is that allergic to barley).
And if your friends get grossed out by water, simply raise your glass, toast to them with a nice cold f*** you, and continue to enjoy your delicious, clear, nectar of the earth.
Also, don't depend on them to bring your drinks. Bring them yourself. If you keep drinking something that gives you that bad of a reaction, you're only destroying your body little by little...acid reflux, poor nutrient absorption, etc etc.