Hi friend,
Welcome to Health Care Magic

You are measuring your height in feet and inches / but your weight in Kilograms!
It is preferable to follow the same units for both. If the height is in inches, the weight should be in pounds.

Your BMI is 17.9
Below 18.9 is underweight.
The normal rage is 18.5 or more and up to 24.9 - in other words, you can be from 130 to 160 pounds.

The first thing to do MUST be to see an internist (medical doctor).
Have a physical examination and investigations (basically blood test) to rule out any cause that can be responsible - like thyroid, sugar and so on.

If nothing is found - which is highly probable - then you MUST see a nutrition specialist (Dietician). Several sessions will be required to be well acquainted with the concepts of macro-nutrients, micro-nutrients, balanced diet, calorific values, so on so forth.

Weight is like bank account. The account grows when you withdraw less (Cut down unnecessary physical work) and you deposit more (eat more).
So for SHORT TERM, eat AS MUCH AS POSSIBLE / AS OFTEN AS POSSIBLE . HIGH CALORIE food / Sweets and snacks in between. Fatty foods add lots of calorie - one gram of fat (oil) has 8 calorie compared to Carbohydrate having only 4 calories. Sweets have lot of ghee and sugar. Supplement with a multi-vitamin pill once a day
THIS HOWEVER IS BAD IN THE LONG TERM / ESPECIALLY WITH INCREASING AGE / THEY INCREASE CHOLESTEROL and so on. Beware. For a short time and in young age, to indulge like this may be excused...

Have you heard of 'flour laddu'? - it is commonly given in parts of South India for the children to put on weight. Finely ground channa dhal (protein) / finely powdered sugar (carbohydrate) / made into balls by adding ghee (fat). Keep the powders ready / make a few daily / eat several of them, several times in a day!

Don't worry. You have the motivation. You WILL gain. A time may come, when you may be overweight and start looking for ways to reduce!

Take care
Wishing speedy recovery
God bless
Good luck

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