Hi, a cancer is the least likely cause. The most likely would be some infection, even a low grade virus in the lungs - which makes for lots of mucus in there which results in low oxygen and then the tiredness and shortness of breath.
Epstein Barr virus can enlarge nodes in chest and neck, too. Also Lyme.
Another cause could be some autoimmunity. A doc would want to know about any personal or family history of odd immune system conditions. E.g. lupus.
Any GP would want a blood test called a CBC. I'd call the office and see if some doc there will order that for you, then you have the results before meeting with your GP. You want to see white cells high (tending to infection).