Here is a quick tip for guitarist survival. Even if you stretch well and regularly - which is something you should do and I will cover another time - we all go through times where our hands, fingers and forearms hurt from playing for hours, moving amps, hauling guitars and speakers, standing or sitting in one, playing position for hours and of course, that dreaded song with the chord that hurts that is played like 30 measures in a row.
I was playing a gig at the Juggling Gypsy in Wilmington, NC and the guitarist in the other band that night shared a tip with me. When your forearms or hands are cramped up, go to a sink and turn the water on as cold as you can get it. Run your arms under the stream of water, from fingertip to elbow, several times. Let your arms get as numb as possible.
Then, switch the water to as hot as you can stand it (without getting burned, obviously) and do the same thing.
What happens is that the muscles constrict a bit from the cold and then they get shocked with the hot water and they are tricked into total relaxation. They go all delightfully soft and gooey; it feels great.
This is an emergency treatment, only. It's by no means a substitute for a solid stretching regimin or proper playing habits. It's simply a way to get through the rough spots.
If you have regular or severe pain, you should immediately consult a doctor, one who has treated carpal tunnel and/or tendinitis.