I've had appetite loss for around a week now, with nausea after eating anything. what's the most likely cause for this?

Is pregnancy a possibility? If not, this may be a case.of gastritis( (inflammation of the stomach lining). Try a soft, bland diet starting with saltine crackers & broth. Advance to more soft solid nonspicy,nongreasy foods. Drink flat ginger ale which helps nausea. Take a medication like omeprazole. If not improving see your MD for evaluation & testing including liver function tests.