Cats showing weight loss, loss of appetite and lack of energy may be caused by the following reasons:

Disease factors: such as Colds, gastrointestinal diseases, parasitic infections, etc.

Oral problems: Tooth pain or oral inflammation may cause a loss of appetite.

Eating problems: Food is inappropriate, spoiled or does not taste good.

Stress and anxiety: changes in the environment, the presence of strangers or other animals, etc.

Endocrine problems: such as hypothyroidism.

Digestive system problems: indigestion, gastrointestinal diseases, etc.