It could be from chemo just as well, and reason we have a metallic taste in the mouth is the body recognizing chemo is a poison, and one of the functions of the tongue, bitterness, is to detect poisons. Maybe try using a water filter to take out most metals, chemicals. Scapegoating by drinking, eating, sucking strong candy, before chemo and radiation, may help prevent learned smell and taste aversions, like with taste, nasuea, vomiting, with treatment. Stick to the familiar, not unfamiliar . Maybe overshadowing the water with something strong added, as mentioned, flavor enhancement by smelling calming oils like orange, citris, lavender, to evoke calm, pleasant memories, before drinking water may help with taste.

Good luck.