You are at the low end of the spectrum for body fat. Normal range is between 22-25%. But it sounds like you have a tiny frame (I am picturing you to be around 5'4 and lean) with little fat.
As for your muscle tone, hard to say without seeing you. Most elite athletes have similar body fat percentages as you, therefore the assumption of good muscle tone, I believe. Perhaps you are more fit than you are giving yourself credit for!