All plastic bottles (even the "BPA-free" bottles) can leach toxic chemicals into the water.
When you fast, your sense of taste usually becomes more sensitive, and your body can also become more sensitive to the toxins in the water.
I try to avoid drinking from plastic containers in general. The petrochemical toxins are hormone disruptors. I have reusable glass water bottles that I use at home and the office, and I use a stainless steel bottle for outdoor activities.