Thank you for the information.
A loss of weight of 13 lbs within a short period is matter of concern.
You have been checked by cat scan or ultrasound studies , which did not show much.
A colonoscopy is necessary to investigate the cause of altered bowel habit.
Though dumping syndrome is associated with rapid intestinal passage of food matter it usually follows after operation of stomach to reduce weight.
You can read about it in this site.
In addition a thyroid study should be taken , as you are having family history.
In thyroid hormone excess there is also frequent loose stool & weight loss.
As planned , do the investigations of lung too, the cause of lymph node enlargement there should be searched for.
I hope this will help. Please press the ACCEPT button if I answered your question. Thank you.