She may be mad at you for a while, she might feel as if you have betrayed her or that she can't trust you. The fact is, by helping your friend to get the help she needs you are being a good best friend. I sometimes feely really down even though there is nothing wrong or sad to feel bad about. Dear Wendy, It's not uncommon for people your age to feel this way at some point. You are probably going through many changes in your life right now and with that can come stress.
Most people do go through a time in their life where they feel down or sad.
Most important is how you deal with those feelings. First, you might want to make two lists - one list of the reasons why you are sad and the other list about things that would make you happier. If these feelings last more than a few days or you find that you are feeling worried and sad more of the time, it would be a good idea to talk to someone. Friends can be great listeners but you might also want to talk to a professional. Counselors and therapists have lots of experience with the issues you may be facing and know how to help you get in charge of your feelings.