As per your BMI you're underweight. The weakness maybe due to malnutrition, I would advise you to do thyroid profile, CBC, Vitamin D, Vitamin B12, etc. if there's any altered values, kindly visit the MD general medicine so that it can be corrected. These following lifestyle changes will help with weight gain and help with your skin: - Don't skip meals, you may not feel like eating but in such conditions you should have the reduced amount in frequent intervals(i.e. 3 square meals broken down to 5 or 6 as you can tolerate) and slowly increase as you get better. - Reduce/ Avoid spicy,fried, fatty, junk and oily food - Exercise regularly (30-45mins/day) - Increase daily intake of water to 3 liters - Avoid/Stop smoking and drinking if you have the habit - Reduce stress through methods such as yoga/meditation/tai-chi.