Never had cramping in my left hand, but I have in my right one. I found I was grabbing the pick too tight. Switching to a heavier pick with a good gripping surface (like the Brain picks have) helped.

Of course I did have another problem with my right hand while working construction years ago. Several months with a large heavy hammer rivalling Mjollnir, with a no-give metal handle, used for making and knocking apart concrete forms for footings and basements for 12 hour days in cold wet weather, 6 days a week, left me with a serious cramping problem for a while. Every morning's first duty was to pry the stiff claw straight and massage it back to life. Lasted for about 6 months after I left that job. Luckily no lasting damage was done (I think).