I am going to chime in here and
guess it is due to
repetative work. The muscles like to change off and when you do the same thing over and over it stresses and congests them.
The cause to my finger pain and base of the thumb pain is the forearm muscle. Check it out. Rub along it to look for a HOT spot. If you find it it needs massaging. Like in Carpal Tunnel you can inflame certain sheaths to the muscles and they tighten and that gives the feeling of swelling....and to the point of actually swelling if the entrapment is enough.
The arm pit area is another that can be troblesome for the arms and create the numbness we get,
In the book I use FM and Myofascial Pain , a Survival manual by Dr Devin Starlanyl...it shows so much and this issue and how to help yourself. I push on the zone and heat it...rest it, or ice it. Going with whatever feels better.
It is one of my bggest weak spots.
Hard to do certain tasks and not push those muscles. Dr P has taught us "arms stay home" meaning try to keep the elbows close to the core....in...it lessens the stress when the arm is outreached.
So hard to re-invent orselves and do things different then eveyone else. Good luck. I know for me, I could not use that fryer and shake the basket at chest height and out above the grease well. Good luck. See if you can get the book on Amazon.com 19.99 and it will teach you some tricks and great information.
Also a strap (tennis elbow band) may help you for now. It supports the muscle not to pull against the forearm structures and lessens some of the symptoms. Pharmacies have em' ask a pharmacist. Wish I could draw on this site to point to the spot where this is probably coming from.
I wish you all the best to help this get under control..........
Nancy B