Given that a cardiac cause has been ruled out, there are a few different type muscle spasms, trigger points, or thoracic outlet probems that could be causing your symptoms. You could see if a physical therapist or chiropractor would take cash payments and go for an evaluation. It is possible given that you described a somewhat sudden onset, that you have your clavicle rotated or a rib/vertebrate out of place.
Or could be a muslce spasm: Here is something I found that kind of sounds like your referral pattern of pain you are describing. http://www.triggerpointtherapist.com/blog/scalene-trigger-points/scalene-scalenus-trigger-points-upper-body-troublemakers/
A lot of times, muscle relaxors or anti-inflammatories need time and steady dosing to be the most effective. Also, sometimes taking extra calcium (like chewing Tums) and drinking lots of extra water can help.
The best position for pain relief can be with a small pillow against your side so that you have a slight space between your upper arm and your chest....using ice in this position is helpful.