Doctors said that my shortness of breath and back pain is anxiety after blood tests and chest xray in Dec. But I think lung cancer, chances?

Extremely. low chance of cancer based on your age, symptoms, and negative work up.
Further evaluation. Lung cancer doesn't appear likely and stress and tension certainly can cause these symptoms. However, if symptoms persist, I recommend further testing such as EKG, echocardiogram, chest cat scan, pulmonary function testing, spine xrays for reassurance. However, your physician is best qualified to advise you.