I have this weird pain under collarbone.?

I have a pain a couple inches under my left collarbone that radiates to my left armpit, and up a little on my left side of my neck. I’ve had this pain a few times. I went to the doctor over it once, they did an EKG and said I have pulled a muscle. I’m a 23 year old female, not overweight (I’m 5’7 and weigh 120 pounds), I do smoke, and heart problems do not run in my family. Could I continue to keep pulling the same muscle? What else could this be? I do have anxiety and tend to think things are wrong when they are not.
Thanks in advance.

Gas.