What Causes Burning Sensation In Left Side Of Collar Bone And Shoulder Blade?

I have a dull achy/burning sensation on the left side only in the area of the collar bone and shoulder blade. This is not an entirely new thing but does seem to be more consistent lately. Occasional twinges also in and under the left breast. No shortness of breath but am awaiting results of an under active thyroid test. Could they be associated?