Sounds like a pinched nerve or a tense muscle knot resting against a nerve. However, whenever someone complains of pain in the left arm you think of the heart. It wouldn’t hurt to call your doctor’s office and just ask. If it’s a tight muscle, massage can help, and yoga, stretching and relaxation exercises (but you already knew that, right? and you’ve been meaning to start a yoga class, right? So go for it!) But first, call your doctors office.
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There is a book that teaches about muscles, pain and referred pain anywhere in the body. The principle is that the muscles get knots called trigger points which make the muscles tight that then press on nerves. It teaches how to get them to release which gets rid of the pain.
The Trigger Point Therapy Workbook by Davies.
I also use a chiropractor and massage therapist as needed.