You aren't fat at all. In fact, you would be considered underweight, which is a more severe problem.
As for a thin layer of fat, it is a secondary sex characteristic of females as they mature, brought on by increased estrogen production. It's perfectly healthy to have!
You should try eating a good sized balanced meal, and get your weight up a little. If you want to tone the muscles, work on that, too, but don't neglect an increased need for food when you work out!
As always- None of my statements are evaluated by the FDA.
My advice is not intended to diagnose, treat, cure or prevent any diseases unless prescribed by a health practitioner. All information is provided for educational purposes only and should not replace the advice of your medical doctor.
(but your medical doctor will probably end up killing you