Hi, The operation that you got done was to lose weight I guess. What happened is that by reducing the area of contact that the food gets with the intestine. This will impair absorption and not allow you to absorb food. This is why you are not able to gain weight. Also, below I have listed all the causes of loss of weight. Go through the list and you can figure out which suits you best. Trapped Wind Constipation Pulled Muscles Kidney Or bladder Infection Irritable bowel syndrome Mid Cycle Pain in young women Kidney stones Pelvic Inflammatory Disease - PID Gastritis (causes more of central upper abdominal pain) Heartburn (also causes central chest or central upper abdominal pain Diverticulitis - more in older people above 50 years of age Pancreatitis Food intolerance Crohns disease Ulcerative colitis Twisted or Ruptured ovarian cyst Ectopic pregnancy Endometrosis Fibroids Infectious mononucleosis or Scarlet fever Coeliac disease Left testicular torsion Bowel Or Intestinal Obstruction Obstructed Inguinal Or Femoral hernia Left lower lobe pneumonia Myofascial pain syndrome Spleen Pain from malaria or other causes of enlarged spleen Lymphoma Bowel Cancer Adhesions Pulmoanry embolism - blood clots in the lungs Hereditary Angioedema Sickle Cell Disease Porphyria POEMS syndrome Eosinophilic colitis Intestinal polyps Typhoid fever - more common in developing countries Go through each one of these diagnosis and then you can decide which of the following sounds like your symptoms. I hope this helps. Dr Vineet