My guess is you're sensitive to one or more of the minerals in the
water
at levels less than max permissible. My grandparents had to haul water
from a county park for me -- I was the only one of the grandkids who
suffered major GI distress from their town water. All of that went
away when the town drilled a new well into a different aquifer, which was
less rich in Fe and Mg.

OP may be able to desenitize themselves to the new water source.

first 90% old water with 10% new water, over time increase gradulally
the new water percentage.

this approach is gow allergies are treated continious increasing
exposure often desensitives the person over long ewnough of a time