Don't stress. My wife has been anorexic and from what I remember of the explanation, your body has gotten pretty messed up with metabolism/ hormones and so it takes time under healthy conditions for it to re-establish the correct balance. It's been deprived of calories/ nutrients while you've been anorexic so it's natural response to receiving calories is to try to store them as it is essentially operating under famine conditions.
My wife has had same thing, going from body image problems and being super muscular to anorexic to what she calls skinny fat. The correct thing to do right now is to eat a healthy nutrient dense diet with a constant caloric excess and as the other guys suggest, to use resistance training to increase muscle mass and some HIIT stuff. You should start producing more HGH and testosterone which will help keep lean, increase your muscle mass and consequently your bodyfat % will likely fall. Do not stress about the bodyfat stuff itself, IIRC the doctor said that it can something like 18 mths before the body starts to operate normally again depending on how long/ severe the anorexic episode lasted.
13% bodyfat is good, as someone else suggested, if your musculature develops you are going to look buff! Going the route of increasing low intensity, high volume aerobic and/ or caloric restriction is going to keep your BMI low, prevent your body raising its basal metabolic rate (which increases with increased muscle mass) and stop you from raising testosterone/ HGH and maybe increase cortisol.
I should add that I am not any kind of expert and I could be wrong, but from everything that I have read and my experience direct and from observing other people, I think this is correct.
The other thing that I would say, if you are not already doing it, you need to get counseling to deal with whatever it is emotionally/ psychologically that has resulted in you becoming anorexic and having body image issues. This isn't something abnormal, most of us these days have ll sorts of issues from such unhappiness and deep rooted trauma in our lives as contemporary society and what are now unhealthy norms all cause deep, deep misery.
I will close with the pointing you in the direction of Jesus Christ as through my unhappy experience of life, I have come to realize that this is really is the truth and that there is a God in heaven that loves you and will heal you from your wounds.