I don't think there's necessarily an eating disorder in play here, but I have to ask the obvious question: where's the mystery here? If you want to gain weight, you do the opposite of what you need to do to lose weight.
If you want to lose weight, you eat less and move more. Then, eat more and move less to gain weight.
You exercise 6 hours a week already? Tell me that you don't know you are in a distinct minority in the U.S. (at least) population - you actually expend energy on purpose with the idea of burning calories and maintaining muscle mass. And you're already a size 2 and you went the 'extra mile' to 'eat clean'? Then you are strictly controlling your calorie intake and output as much as you can.
So, then you ask: oops, I'm losing weight. I'm eating clean and I don't like what it's doing, especially losing breast size. So, where's the mystery?
You can 'eat clean' and just eat a lot more calories while still eating clean - lots more calories than you are eating now. Simply add more fats to your diet and you are practically there already.
The question suggests an eating disorder, but being a size two, you may simply be a small person and it's not an eating disorder at all. In which case, please forgive me for even suggesting the possibility. But the answer is still clear: you just eat more calories. You can cut down on the exercise, too, if you think that would help, but exercise is so beneficial, you don't need to do much other than switch from Jazz exercise, etc., to simple walking activities.
If these things don't work, and fairly quickly (it's easy to gain weight by eating more calories, unfortunately), then your doctor would be interested in finding out why!