Hi,
Your ideal body weight range should be 50 - 55 kg, means you are underweight. The reason behind this need to know in details, like whether your metabolic rate is high or any other malabsorption is there or not. There are many causes behind this but which is specific one that need to know.
Next Steps
If you want you can consult with me through this practo aap.
Health Tips
You should follow protein rich nutritious diet. Include egg, chicken, cashew, almond, dates, soyabean, milk, dahi/ yoghurt, sprouts, fresh fruits and vegetables. You can have small meal but calorie rich food, so you don't need to take lots of food at a time but it will be calorie dense, Ex - badam milk, moong-massor khichdi, veg stuff omelette etc.