What Causes Pain In My Armpit Radiating To My Neck?

I have been having pain under my left arm (armpit) for a few days and now I am starting to feel it in my neck. Last night it was kind of hard to breath. I felt pressure in my chest. I do not have a fever. My family has been having a lot medical issues and I am trying to decide if I really need to pay the 40.00 copay and go see my doctor or if this is just somthing silly that will pass.