Formula-fed babies and hard water areas?

So we went to visit family over xmas and ILs are in a hard water area. By day 3, LOs had an upset tummy and sour smelling diarhoea. Went from 1 poop every 2-3 days to having 2 a day. She has no other symptoms that we can tell. Wondering if it could be the water? Does anyone have experience with formula feeding in hard water areas or travelling to hard water areas?

Edit: we follow UK formula guidance. So cold tap water boiled and mixed with formula at 70°c then cooled using cold water bath to feeding temperature. We only noticed as the hard water seemed to leave a film on the baby bottles when washing and water was rather cloudy from the tap.