Please tell me since how long you have this problem?
Any history of redness at the site of pain?
Any past history of herpes infection?
Any history of fever?
Thanks for info after your history it seems that you have nothing but just a muscle spasm that is giving rise to this pain and specially when you bend down you feel more pressure over chest you have no cough,fever or redness along with all these symptoms muscular spam is the likely diagnosis i will suggest you to take muscle relaxant tab tizinidine 2 mg three times a day along with tab brufen 400mg three times a day for 5 days and do physiotherapy of shoulder area you will get better and dont worry.
Well you should take tab brufen 400 mg three times a day or tab aleve 220 mg two tablets twice a day and apply heat that will be soothing in place of tizinidine you can apply locally brufen gel before going to bed in addition to that dont lift heavy weight with this arm,dont put strain over arm and physiotherapy is very helpful.