Water PH changes

I have one of those water PH testers where you put a bit of water in a vial, add a few drops of solution and then compare the color to the little chart of the side of the solution. When I use nutes it acidifies the water big time so I add a bit of PH Up until it's barely green...in the 6.5 range.

What's been happening is 2 days later when it's feeding time again, I get out the vial, and the water I tested from a few days ago is blue! Pretty alkaline.

Why is this? I assume the PH level when I apply is important, but does it then change?

if your useing city water then most likly the clorine is evaperated cousing the acid to lower but you had already raised the ph.. try letting the water set for a day then mix your nutes and then test ur ph..