I am getting thin and losing fat and muscle day-by-day for the past six years. Why?

Hi doctor,

I am a 23-year-old female, of height 152 cm and weight of 36 kg. Why am I losing weight? I am getting thin and losing fat and muscle day by day for the past six years. Now I have lost so much weight and developed malnutrition, anemia and became so skinny that all my clothes do not fit me anymore. All of my shirts, pants, caps, and hats have gotten loose. For the last four years, I am living a sedentary lifestyle. I always stay at home for 24 hours and occasionally go out also. I do not do any exercise or sports or extra physical activities but still, getting thin daily. Whenever I walk or do some other physical acts (stair climbing, running, etc.), I lose some inches but never recover it. Sometimes I experience digestive issues, but it gets cured after taking multivitamin tablets. I can perform my daily chores and physical activities only after taking my multivitamin or multimineral supplements. I gasp after climbing stairs, and cannot walk properly because of weakness. I started eating four to five times daily, and I poop two to three times daily.

My periods are not regular; there are two to three months gap. I am a virgin with low BMI and a nonsmoker who occasionally travels in a two to three months gap for the last four years. I have lost so much fat, muscles and have become so thin and weak. My face and head are shrinking too and has become so small. It looks like my face and body are melting day-by-day. Everything was normal with bowel gas in the whole recent abdomen ultrasound report, and PCOD in the right ovary was seen.

Why am I losing weight, and why are my muscles getting thin and weak day-by-day?