Im 22 what are rare causes of appetite loss? is cancr a common sign?

No: Cancer is not a common cause of appetite loss in young people. Common causes are depression, anxiety, gastro-intestinal conditions like irritable bowel syndrome, gall bladder problems, some medications and viral infection of the liver (hepatitis). Persistent loss of appetite should be evaluated by a physician by a detailed history, examination and some investigations.

Need to examine: It is not feasible to provide a meaningful opinion without taking additional history, physical examination and may be some tests. It would be prudent to see your doctor and discuss if you may be depressed. Counter-intuitive as it may seem, exercise may help. Start with low intensity exercise such as walking and gradually increase the intensity as tolerated.