Surely your weight is too low,for this
Firstly, you find out the reason of your low weight, not eating well or hereditary or any health related problem like indigestion, liver problem or any allergies with particular food item.
Secondly, start taking high protein diet with 8-10serving of cereals
2 servings of pulses or paneer
1 servings of white meat products
700 ml of toned milk products
5-6 servings of fruits and vegetables
12-15 glasses of fluids and
6-8 hrs of sound sleep with 50 mnts of physical activity of your choice daily to improve your health.
Make sure to find the cause first to eradicate the problem. If undergone any testthen also tell me in details to guide you better.