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