Your post is kinda confusing. What surgery are you due to have? Taking any medications as this time/any medical conditions?
Welcome to the MedHelp forum!
If you have cut down on fat and carbohydrates then this can explain the weight loss. There are many causes like worms in stool, depression, anorexia nervosa, AIDS, cancers especially colon cancers, drug abuse, infections and loss of appetite. Loss of appetite can again be due to certain medications, drug abuse, depression, AIDS, acute and chronic infections, cancers and hypothyroidism. Either you do not eat well or there is increased metabolism as in hyperthyroid states, HIV, cancer etc. Poor absorption as in irritable bowel syndromes and malabsorption syndromes too is possible. Malabsorption syndromes are seen in intestinal tuberculosis, HIV, tropical sprue, parasites in stool and Whipple’s Disease. I have enumerated the possibilities that come to my mind right now. This does not mean you have any or all of them. You will need to consult your doctor regarding them.
Hope this helps. Take care!