One day, my friend told me that he got neckache after merely 30mins playing pool. On the other hand, I've gone 8hrs straight without being reminded that I still have a neck. I didn't understand his experience at the time.
Then years later, I hurt one eye (healed in 3 weeks, not permanent, but pretty bad). So I had to lean out in a certain way when shooting. In doing so, my neck stiffens subconsciously. And sure enough, in 15mins, my neck started its intro.
If this is similar to what you experienced, perhaps it might be your stance.
Only the shoulders should be held. The other joints would produce the arts of shooting. As long as the cue is centered under the line of aim (centered between 2 eyes, or under 1 eye in some situations), then the neck needs not be stiff.