Hello, There are various reasons for being underweight - A person may be underweight due to genetics, metabolism, drug use, lack of food , eating disorder, or illness (both physical and mental).
As per ideal BMI parameters, you are normal weight.
Next Steps
If you feel weakness or lethargy, please contact professional dietitian who can help you come up with a plan for foods you should be consuming, how many calories, ratio of macro and Micro nutrients, and monitoring your progress through practo.