Hi, Welcome to Health care magic forum.
As you describe, you are moderately low weight, and you can gain weight easily.
I advise you to consult a physician for diagnosis and treatment .
If the reason is genetic , absorption, or chronic disease problem they are to be treated first.
I usually prescribe to my patient with such symptoms injections of anabolic
steroids periodically.
protein supplements, enzymes, and multivitamins.
Take more of green leafy vegetables, pulses, sprouts, and protein rich foods.
Wishing for a quick and complete recovery. Thank you.