Don't be scared. If you were having heart problems you would certainly know it.

The problem is likely muscular, or maybe a trapped nerve, or could even be pleurisy (inflammation of the lining of the lungs) or something like that. Does it hurt when you breathe in and out? Especially if you get a stitch-like feeling? That could be pleurisy or even heartburn and is not likely to be something you need to be scared of, but if you get it for a few days have a word with your GP; you might need to take ibuprofen (for pleuritic pain) or an antacid (for heartburn).

Does the pain come on when you move your arm into a certain position? Maybe you've been holding the steering wheel in an odd position while driving for a long distance, or are sitting in a bad posture at your computer, or you tried to lift something that was too heavy for you? Maybe you banged your shoulder and thought nothing of it at the time, but days later you've come out in a bruise.

Breathing difficulties may indicate pleurisy or chest infection, but ask yourself if you're feeling especially anxious, as anxiety is nearly always the problem.