She probably has/had bulimia.  As someone with an eating disorder I looked through her whole instagram. There's one of her with an 18-19 BMI and she went to Oliver-Pryatt which is a residential eating disorder program. In a comment she said she had tried recovery 4-5 times for various months.  It's not uncommon for eating disordered people to end up looking like her. I met a girl at a treatment program who used to be anorexic and was eating pizza and other shit with that body type.  I don't know how they do it.