I have had an issue in my left arm for 3 months. It starts a dull pain in my thumb and radiates up to my collar bone. Fells like a nerve. Not sure?

Agreed, nerve pain. It does sound like nerve pain. Most likely the irritated nerve is in your neck with radiation to your collarbone area and along the nerve pathway to your thumb. Less likely the nerve irritation could be in the arm itself. An orthopaedic surgeon should be able to get you a firm diagnosis. It may require some testing after you are examined.