Hi,
Welcome to the ehealthforum and I am really glad to help you out. The first possibility that is to be ruled out is a cardiac cause like angina or myocardial infarction. Get an EKG and ECHO done from a physician to rule it out and these should be done urgently. Once cardiac status is clear, we can think of other possibilities also. The other possibilities can be thoracic outlet syndrome or brachial neuralgia which may cause such symptoms.
It is very difficult to precisely confirm a diagnosis without examination and investigations, and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care.