Is it okay for a 13-year-old girl to have a BMI of 16.1?

So… I'm entering recovery for bulimia next week and I'm 19 right now. I've been bulimic for about a year. The last time I had these obsessive thoughts about control and weight loss was when I was 13 and I was consuming no more than 300 calories a day. I still exercise up to 3 hours a day, but I DO eat now (keeping everything down is the main issue…) and I've been in the low end of a healthy weight range for a while with a slight athletic build. I'm just wondering if it's possible that I might have a history of anorexia that was untreated and lead into my current bulimia. I'll be sure to bring it up to my therapist.