If you think it will help you, confide in your friend. It seems that she is concerned since she cared enough to ask. Since you won't see each other over the summer you could stay in contact over the phone or internet. Just tell her you need a friend. But do not depend on her as you would if she were a doctor. Seek professional help. I don't know your history but talk to your parents about speaking with a nutritionist or a counselor before things spiral out of control.
But I think we all feel attacked when people ask...But I don't think people mean to hurt us intentionally. My mom has no idea but my bf's parents have asked me straight out if I'm bulimic. Or if I'm eating enough. It hurts cause they can see right through me. I've been binging lately and gained 10 lbs. I've been avoiding them because I know the first thing they will say when I see them is: Have you gained weight?
Anyway, I'm going to the doctor soon (I've waited too long- because the ED is etched into my brain). Seek professional help but remember it's okay to talk to your friends and family. Don't wait to it's too late.