you're too tense.
for the crochet hook: go to walmart, in the stationary department pick up a package of those soft foam grips for pencils if you're using smaller hooks, I used to put a cotton ball around the middle of my hook and then use scotch tape to hold it in place (i gripped my hook way too tightly).
for the shoulders and neck, dont' sit hunched up. find a comfy chair (i stole my dh's ergonomic desk chair, it's way comfy or i'll sit on the couch with my feet up on the ottoman and have the pillows adjusted behind my back with a regular pillow across my lap to help support the weight of the project i'm working on). get up every 20 minutes or so and do neck and shoulder circles to loosen them up. I made the mistake of doing a crochet marathon for a week... i didn't sleep, didn't clean... only got up to refill my drink, bathe, and use the restroom...
Pulled a bunch of muscles stretching after staying curled up on the couch to do the afghan.