I have been having left shoulder, armpit and neck pain. it sometimes radiated down the whole arm. what can it be? it is constant!

See physician: This could be a number of things, including nerve root irritation from your cervical spine, a brachial plexus injury from trauma or tumor, vascular, or even just referred pain from a shoulder injury. I wish i could be more help, but with a broad spread of pain and limited history, the best recommendation would be to see a doctor. Make sure to ask about above listed issues and see what they think.