Hi, Welcome to health care magic, I have read your query and understand the problem. This is can be irritable bowel syndrome or gastroenteritis or uncommonly inflammatory bowel disease like chron s disease or ulcerative colitis as you have lower abdominal pain and loose significant weight. You should undergo routine blood, stool investigations with ultrasound of abdomen and upper GI scopy. Treatment is mainly based on the underlying cause but symptomatic treatment can be taken like antibiotics, anti-inflammatory or probiotics or prebiotics. Consult gastroenterologist for complete medical history and clinical examination with some routine and specific tests to rule out the possibilities one by one . Thanks and regards,