What would cause a dull burning pain on left side of chest above breast line and below collar bone? 20 minutes so far. Pcp says heart ok.

A couple of things. There are a few things that can cause that symptom. Regurgitation and reflux probably being one of the more common ones. If your PCP has ruled out the heart and there's nothing else except that prolonged burning then, I think it less likely cardiac. You may also have an entrapped nerve twig supplying the pectoral region or skin (perhaps trapped by the clavicle)...sensory neuropathy.