Just a thought that possibly your wrist is wrist side down on a table. Maybe your grasping the pen tightly (I know I do) and there is pressure from this. Take lots of breaks and every once in awhile stop and give your hand a gentle stretch and shake. Thoracic Outlet can cause many symptoms because of immflamation, nerve compression etc. All kinds of symptoms!! Take Care and try to pace yourself and respect your pain. Meaning don't wait until pain levels sky rocket or increase before you take a break and get away from the activity for awhile.

Just some thoughts, I'm sure some others will give you some helpful insight soon.