17 y/o post appendectomial ascites (small amounts), enlarged lymph nodes, fatigue, nausea (sometimes), mild abdominal pain, history of gastritis/ulcer?

Complex issues. Ascites is not normal , it should not be present . Enlarged lymph nodes if in the gut are not normal either . Be sides you are symptomatic . I would recommend to see your pcp , he /she will know what to do ( where to refer you to). Good luck.