I'm a 48yr. old smoking female, experiencing tightness and heavy feeling right under both shoulder blades in my back. made an appnt, but is it cancer?

Dont wait: This could be an emergency. See someone right away er if needed.

Could be anything: Or could be nothing. Nobody can say anything on the basis of just those symptoms. You need a more complete history taken, a full general examination with special attention to chest and abdomen (shoulder pain can reflect gall bladder disease, pancreatic disease and other things), and probably some tests, depending on the results of the examination. Stopping smoking is best thing for now.