Welcome to the MedHelp forum!
What is important is to find the cause, which probably you have done the exercise. However please go through this list and see if anything was missed. 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.
If all causes are ruled out, consult a nutrition expert and build up a good diet program. A physical fitness trainer then can help you gain some muscle mass. Do not go for so called weight gain pills as they make you weak and play havoc with your endocrine glands.
Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!