Thanks for your query.
I will try my best to provide you with best possible professional recommendations to your questions.
It might be from sprained muscles (as a result of excessive workouts , pressure or stretching of the arm muscles ) , pinched neck nerves (many times pain in the neck or a pinched nerve there can lead to pain in the upper left arm) , heart problems (many times the first symptom of a heart problem is pain in the left arm ), angina
and poor posture (this can lead to muscular tension) etc.
If it is due to muscle spasm it will reduce with in week or so, I advice you to reduce the work on that arm and also you can take hot fomentation or pain killing spry can be used.
If does not subside then i advice you to meet orthopedic surgeon for examination, to rule out any pathology in neck and also do an ECG and show that to physician.
Yet again, I do hope that you have found something helpful and I will be glad to answer any further query.