The weigh of 48 kg for a height of 5.7 ft is really less than it should be.
First thing I would suggest you to take a protein rich diet and have regular exercise. Another thing the caloric intake should also be increased.
Apart from this try to find out if some thing is wrong with the gastrointesinal tract. If you are suffesring from some sort of worm infestation or some type of malabsorption
problem or anything abnormal related to this.
I would suggest you to start taking protein and calorie rich diet with regular excercise and consult some doctor to have a proper check up regarding any problem with the gastrointestinal problem.
Thanks and take care.