What could be the cause of good appetite but slightly loosing weight?

Count your calories. Weight loss occurs when you burn more calories than you take in. This can be due to maldigestion or malabsorption of food you're eating, filling up on a lot of low calorie foods (proteins & carbs have 4 calories/gram; fat has 9 cal/g), a faster metabolism from a variety of disorders, vigorous exercise or training, stress reaction. Alarm symptoms or ongoing weight loss deserves medical evaluation.
Check=HYPER-thyroid. Unintended weight loss inspite of a 'good appettie' may be a sign of mild hyper-thyroidism. This is a condition where the thyroid gland (at the base of the neck) produces excess thyroid hormone causing your 'metabolism to speed up'. Other signs may be palpitations, shaking , tremors or 'jittery'. An exam by your physician and labs are necessary to make the diagnosis.