Nobody here can diagnose you, because most of us aren't doctors, and even those who are can't diagnose you over the Internet.

However, I would be concerned, too. While many anorexics exercise, often to excess, one doesn't have to exercise to be anorexic. You're not eating enough.

I don't know what you look like, nor do I know what qualifies one to be medium frame vs. large frame, but your therapist probably knows better than you do. You do seem rather obsessed about gaining weight and your diet, especially since you don't want to gain muscle--which is generally a healthy thing--because you'll gain pounds. It won't be fat pounds, though...although you need some fat on you as well.

I think your therapist has reason to be concerned.