Hoow long have you had the pian? if less then 2 months then it could have been a simple pulling of muscles or a tear in your rotator cuff. If its been longer on and/or then it could be anything from 0stiophites (bone spurs) to degenerative disc disease in the neck and/or shoulder. but because you say the pains are not constant, it could be lungs, gas pains or just being muscular skeletal, the only way to know for sure is to go into see a doctor, additional questions and tests, I would start with a simple x-ray of the shoulder and cervical spine. this will also give a high level picture of gas, muscl strain etc... you should start there with a doctor and some blood work, ans some medical history. I would not worry to much until you have some evaluations completed.
What you are describing is classic symptoms of a heart attack, have doc check it out or go to E.R. just to be on the safe side.