Hi, Welcome to Health care magic forum.
As you describe, you are under weight, as a whole, and the decrease of 7 kg is a temporary change, and you can gain the weight after recovery.
For gain of weight from your normal weight you can gain medically.
I advise you to consult a physician for treatment .
I usually prescribe to my patient with such condition injection of anabolic
steroids periodically,
protein supplements, multivitamins,iron and enzymes.
Take more of calories, fats, fries, ice creams, also Take more of green leafy vegetables, pulses, sprouts, and protein rich foods,to have good health.
Wishing for a quick and complete recovery. Thank you.