Hello You mentioned that your age is 18 years and weight only 39 kg , so weak and this is concerning if you are male and height is 5 8-9 . However it is very difficult to predict and y disease or reason of this weakness but I recommend you please get in C B C ( hemoglobin & leucocytes count), E S R ( chronic infection ) calcium level , elisa test ( hiv test ) . If all these tests comes normal take high protein diet ( cheese ) , protein powder . Do regular exercise and also take appetizer stimulant ( cyproheptadine + sorbitol + tricholin ) as I advise my patient . Still no relief then consult a psychiatrist / Physician an get their opinion.