I've also made a saline solution of NaCl 0,9% pH 9 and after 3 months it came down to pH 5. I don't understand why the pH came so drastcally down. I tested for any contamination and the results were negative.
On the assumption that the sample vial was open, we expect the effect of the oxygen gas to be clear on the hydroxyl ion to change the solution to the neutral formula (pH = 7), but reaches to 5, this is unexpected, unless the conditions of storage of the sample in an environment of high concentration in Nitrogen, Sulfur Oxides or Carbon Dioxide, whether or not the storage environment.
It may be that the container is not tightly sealed, absorbs carbon dioxide from the air, or the bottle is not clean, and some substances are dissolved.