Can drinking tap water cause stomach pain? I don't have this problem when I drink bottled water, but I do with tap water.
I've never heard of this but if you are consistently getting the same result, then I guess it's possible. Very often things affect some people more than others. For example, bananas are on the list for reflux but yet I can't eat them. It might simply be that the water fills your stomach and is washing back up and causing a reflux-like reaction.
Here are some links you might find helpful:
5 Drinks to Avoid with Acid Reflux
Heartburn Trigger Foods
7 Symptoms of Acid Reflux