17 y/o, history of ulcer/gastritis, mild nausea, fatigue, swollen lymph nodes, indigestion, mild abdominal pain and tenderness could it be stomach cancer?

No, Not likely. Stomach cancer(gastric cancer) is an uncommon condition and it primary occurs in adults and older folks. You should go see your doctor who can easily treat your gastritis. .Avoid alcohol and aspirin/motrin like drugs which can cause gastritis. Antacids may be helpful to relive your symptoms. But you need a primary care doctor who can help you to find out the nature of the problem and treat it.
Unlikely. At your age it is less likely. Anyway, you need to get a thorough evaluation by your doctor to get the correct diagnosis.