61 yo male. Lost 20 lbs/month. Extreme fatigue. Stage 3B nsclc stable since 6/30/17 and now "critical" stenosis of aortic artery. Possible cause?

Unknown. I assume you have "critical" stenosis of the aortic VALVE (not artery). Aortic stenosis can occur in patients with prior bloodstream infections & in those predisposed at birth/childhood because of a bicuspid valve or rheumatic heart disease. In your case, however, the weight loss & Stage 3 NSCLC seem to be more pressing concerns. Ongoing weight loss isn't considered "stable". Check w/ your doctor.