Hi i am Dr Ahmed Aly thanks for using healthcaremagic site , I had gone through your question and understand your concerns .. In my opinion i dont think you need to worry as heart diseases or heart overload may send a kind of referred pain to your left shoulder and very rarely towards your back as long as your pulse oximiter is good with normal pulse and oxygenation . Actually this could be a kind of a muscular sprain or spasm that is relieved by hot massages with topical gels containing anti inflammatory and muscle relaxant actions from OTC . for my patients i may suggest X rays and MRI for proper evaluation and management . Please click THANK YOU and consider a 5 star rating with some positive feedback if the information was helpful. Hope the above information helps you, Any further clarifications feel free to ask.