I always think it is best to see a doctor if you have a sudden onset of pain that you aren't sure why it is happening and it persists. So, I'd give your doctor a call and go on in and be seen.
Does it hurt when you breath in? Could you have something like pleurisy? This involves the pleura which is a membrane that surrounds your lungs and it can get inflamed and be painful.
Or have you had any type of issue or injury or work out that could explain the pain?
I would call your doctor or please provide some more details describing what is going on and we'll try to help. If you do see the doctor, please let us know what they say. And if you think you are having a cardiac issue, please go to the ER.