Hey guys, I was just wondering if you could give me some advice

I'm quite an experienced guitarist (maybe 7 years or so) and I've always had a problem which I assume would just go away with time, but it hasn't......so I finally decided to ask for help

My problem is this, when I play songs with a lot of power chords, rock you like a hurricane and things like that, I get cramp in the muscle between my thumb and the rest of my hand....now I assume this is because songs like this never let up in terms of pressure on the neck, is there any way I can stop this happening?
on certain songs you can use drop tunnings thaty might help your hand out
You can strengthen the muscles there with one of those....squeeze-ee things you see in like 80's movies. I don't know what they are called. You squeeze it.

And you might just have like...tension issues. Try massaging the space between your thumba nd hand.
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maybe even squeezing a stress ball would help oh and warm up before you play a song that you know gives your hand a cramp
it's the fretting hand, and it's wierd, I can play pretty technical stuff (relatively) flawlessly, Children of Bodom, Metallica, Lamb of God etc. but when it comes to really simple stuff where you can't release the pressure on the neck at any point my hand cramps up......even things like all the small things make it happen, it's really annoying

Thanks for the advice, do you think squeezing a tennis ball would help? or will that be too hard?