goldfish suddenly skittish

I have a single GF, about 2 1/2 inches long, which has been very
friendly ... usually watching whoever is in the room, doing a "feed me
dance" if you draw anywhere near the tank.

Suddenly today it is very skittish ... darting around, runs to hide if
you come near the tank.

I had done a 20% water change yesterday, so retested the chemistry:

Ammonia = 0
Nitrites = 0
Nitrates = 10 ppm

pH had been holding close to 7.0 for past couple weeks, but today it
shows pH = 7.6 ... but that is the top reading of the kit (Aquarium
Pharmaceuticals), so could be even higher.

I did add some Doc Fishwell's Aquarium Salt with the water change ...
could that account for the pH change? Might it account for the behavior
change?

Tank is 5 gallon, with a little gravel and a Penguin Mini bio-wheel filter.

Any ideas what is going on with my fish?

Thanks,
Dennis