When I moved to Poland after living in the UK I once tried tap water and was ill for a few days! That stuff kills!
It's because water chemistry differs a lot in different regions (of country, continent, globe) and your organism have to get used to it... most probably you will have stomachache in first few days, but then everything should be ok. The most important... even though you had stomachache after drinking tap water it doesn't necessarily mean that the water was bad (polluted).
Do people drink water from the taps in Warsaw?.
Personally I don't know anyone who drink tap water and I'm thinking here about Poland as a whole... some people boil it, but most of us just buy mineral water in shops :P