I Am Wondering If I Am Having A Heart Attack..

I am wondering if I am having a heart attack.. I do not have the tightness in my chest.. I do however have a weakness dull pain in my left front shoulder I am concerned I do not have much trouble breathing but I am worried could it be a gas bubble? I am overweight and 40 years old .. about 285 and I walk a lot for work on my feet a lot.. I have tried reading different articles on the web about heart attacks but none of the symptoms I am having are definitive can you offer and diagnosis