Maybe gastroparesis. Feeling full after only a small amount of food is called "early satiety" in medicine. If it's been going on for a long time, it can result in weight loss. It is a red flag. The common cause is gastroparesis, a condition when the stomach is slow to empty, often seen in patients with diabetes. other causes include infections, autoimmune disease, neurological disease, and cancer. Go see your doctor.
Seek attention. Early satiety or feeling very full after eating even small amounts, may indicate a serious stomach condition. See your doctor immediately. You may require an endoscopy to see if you have an ulcer in your stomach or a space-occupying lesion in your stomach, such as a tumour (either cancerous or benign). Don't delay in seeking medical attention.
Different causes. Stomach inflammation such as gastritis or ulcer conditions can cause an uncomfortable feeling. Also gallstones can cause a full feeling after eating smaller amounts of fatty foods. Some medications may disagree with people and cause an upset feeling. Best to see your doctor for evaluation.