Dull but acute upper left back pain after smoking. No cough, sputum, shortness, heaviness. Not disc or muscle. X-ray neg. Quit cigs today. Advice?

Ibuprofen. That occasionally happens with smoking or right after quitting. You may have had some inflammation, an infection, or fluid behind your lung. It should go away on its own, especially if the chest x-ray was negative. But follow up with your doctor anyways of it persists more than a few days.
Impossible to say. Congrats on decision to stop smoking, but there is no way anybody can diagnose you just by your symptoms. Most back pain is musculoskeletal - strains of muscles, tendons or ligaments. So, statistically, that is what is most likely. A full examination of your back, as well as a general physical examination of the chest, heart and abdomen is needed to come up with a good guess, then more testing.