What Causes Throbbing Feeling Under Collar Bone?

For the last 2 hours, I have felt a throbbing feeling just under my collarbone on the left side - is this very serious? Or can I wait until morning to call my general physician about this & make an appointment. What does this sound like - perhaps? I am 61 years old. (I have a slight arhymia for which I take 25 mg Atenolol each day).