For 3 months I've had pain in my left thumb. Now raitiating up arm to behind collar bone. Dull ache. Leaves my arm weak. What could this be?

Herniated Disc . Your symptoms are consistent with a herniated disc in your neck, specifically the 6th cervical nerve. You need to be evaluated by an orthopedica surgeon.

Related Questions

I have a dull pain starting in my left thumb radiating up my arm behind my collar bone. Seems to get worse as the day goes on. Leaves my hand weak?

Several things. Pain that severe needs to be worked up by your doctor in person. He/she will listen to you, perform an examination and possibly order some tests to see what's wrong with you. Read more...