How to *correctly* measure the PH of my nutrient solution?

Hey guys, awesome site here. Im new here obviously But ive been lurking for a while...

Now this may be a simple/dumb question but here goes.

After obtaining my water and thoroughly mixing in my nutrients ( i actually shake the whole bottle to mix the nutes - its only 5L ) , and letting stand over night i come up with a PH of 5.9 ( i've got a pretty good Essentials PH sensor, correctly calibrated etc )

Now in the back of my head i thought , is this the actual PH (now its settled over night ) or should i mix it again before taking the measurement.

So i decided to shake the bottle again, and the PH dropped from 5.9 to 5.5, just from stirring up the mixture.

Any help/ideas/contributions ???

Anyone? Could do with knowing as i need to feed the babies

id stir it then check the ph rite away , either way that ph range is ok for hydro. if its soil ull need about 6.5-6.7

Thanks
Yeah i'm growing my plants in coco coir , so i could do with a ph a little higher, im aiming for about 6.2.

Right i rechecked the solution today its still 5.5, gave it some serious stirring and it stayed at 5.5.
I guess i didn't aggitate the solution enough so it wasnt mixed ( even though it had been stood for a day and shaken twice ! )

Other than that, not too sure. Probably me being dumb or something. Thanks for the help though.

I think the shit, constantly changes. Accurate reading at the time of useage.