How quickly can chicken stock turn sour?
Boiling stock for a long time will not make it sour.

In my experience, high protein food will not go sour when it goes bad. It goes putrid. Sourness requires sugars. Either way, I agree that your stock would not have been given sufficient opportunity to go off.

I am forced to blame experimental error. Perhaps your utensils were not as clean as you thought. Maybe you just ate a spoonful of honey and your tongue was uncalibrated.