Can someone drastically and suddenly lose weight naturally ?
This generally doesn't happen.
In most cases it is due to some metabolic problem. Diabetics loose weight. Or people with infection or some chronic disease.
It would be best to see a general physician and get a full body check done.
Once you know why it is so then proper steps can be taken.
God forbid if anything abnormal is detected I would suggest you take natural ways to cure it.
Except organ failures everything else is curable by diet and lifestyle.
Do not worry. Just find out what it is.