i've acutely found odd pH changes in my water bottles.

I let them all sit for at least 24 hours at about 30 C
The water is all in 4L mil jugs (but ones that never held milk)

So if i pH my water to 6.4 and use it. a few days later i test it again the pH often goes up above 7. I find this very odd. Is it related to the plastic? is plastic basic or something. or could this be due to the reasons above:
-acids reacting slowly
-pH of water just changing with time as blue bear said