My Husband Has A Lump Under His Right Arm And

My husband has a lump under his right arm and feels attached to the back of the muscle when you push on it he gets a tingling sensation in his thumb and fore fingers. He does alot of driving as it is his job and the lump appears to go down after rest but not totally. It was almost golf ball size last week but is now probs half the size, but driving seems to aggravate it. Any ideas please