I've been eating a low fat diet & have lost 20 pounds despite eating constantly. Is 7-9% calories from fat too low? Why do I never feel full?

You don't feel full. because your are eating low fat. Good fat is good for you and creates a feeling of satiety. Olive oil, coconut oil, butter, etc. are good fats. Bad fats such as trans-fats found in lots of prepared foods and cakes are bad. Check w/your doc, but low carb and higher protein is a good choice and you won't feel so hungry. Peace and good health.