I am 20 used to smoke a lot now having chest and back pains could it be lung cancer?

Unlikely: "from 2006-2010, the median age at death for cancer of the lung and bronchus was 72 years of age. Approximately 0.0% died under age 20; 0.1% between 20 and 34; 1.1% between 35 and 44; 7.8% between 45 and 54; 19.6% between 55 and 64; 30.5% between 65 and 74; 30.1% between 75 and 84; and 10.8% 85+ years of age" http://goo.gl/5mixx risk decreases with time. http://goo.gl/vd4ue.

Unlikely: But possible, twenty is on the young end! good to quit! need to have full history and exam to sort out your situation.