This was the story. I left home going to the nearby province. I was in a hurry that I forget to bring a bottle of water to drink. I took a stop at gasoline station and bought a liter. The Select Purified Drinking Water. It is not a popular brand but it tasted like most variants out there. And I have not seen one of these sold by street vendors.
The taste was perfectly normal after getting out of the chiller, consuming one third of the contents and before I left it in car and the car was parked under the heat of sun for several hours. After, it tasted like the one I usually bought in bus.
I couldn’t explain and describe clearly how it taste. It was not bad though. It was something that I only experience whenever I am in a bus going home. As for the cause, perhaps the heat was forcing some polymer components into water.