I agree that you should see your doctor and have some tests run. I had EKG and other tests, found out mine was all stress/anxiety related. Sometimes it's hard to tell if it's serious or just stress.
I've had the same pain related to anxiety and stress. I don't necessarily have to be thinking about something specific at the time, but can have underlying anxiety stress (usually financial and work related). It's caused pain in my chest, shoulder, back and neck. Aching, tightness and some sharp pains.
Pain in the left side of the chest that radiates into the neck could be from coronary artery disease, so you should see your doctor (and/or a cardiologist) for evaluation. This will likely include an ECG, echocardiogram, and possibly a stress test. Other things could be heartburn, musculoskeletal pain, or pleuritis (recent viral infection?)
Heart attack symptoms-- may be a heart blckage. get it cheked immediately. It could also be gallbladder issues