[Eternal]

Well to be anorexic you have to be under 15% body fat. The other reason why you have to wait 3 months is because an eating disorder is a serious thing, it's not just based off your actions, but it's a state of mind too.
For example, if a girl eats half a cake then purges it the next day, she could go back to normal eating habits after that. She doesn't have an eating disorder, she just made an unhealthy choice for a couple days. Now if every day she counts calories and purges for 3 months, then her ways of thinking about food and eating is abnormal and unhealthy so it would be classified as an eating disorder.
However, you could have EDNOS which stands for Eating Disorder Not Otherwise Specified. This means you may have some habits of an eating disorder, but you do not fit all of the criteria.