Tap water has a lot of chemicals and crap in it (how much and what type depend on where you live). So it actually probably always tastes bad, you might just be used to it.
Water mixes with CO2 (carbon dioxide) in the air to make carbonic acid. It's mildly acidic so it would never burn you, but it does taste weird.
If you can, try distilling some tap water, you will be amazed (and disgusted) at the weird smelling brown crap that comes out of it!