To address your symptoms safely, please consult a healthcare provider for a proper evaluation. Your back pain and weakness may stem from nutritional deficiencies associated with being underweight. A doctor can run simple blood tests to check for vitamin deficiencies or anaemia. To help improve your condition, focus on a nutrient-dense, high-calorie diet. Supplements like Vitamin D3, Calcium, and a B-complex multivitamin can help strengthen your bones, support muscle function, and boost your energy. Gentle stretching and core-strengthening exercises can also help relieve back pain. Please avoid taking specific medications or high-dose supplements without a professional medical recommendation.