It may be the age. It also could be anxiety. As a teenager who knows what you mean all too well, I suggest checking it out. Her invtroverted-ness may be her coping mechanism. I know that I was similar to your daughter. I would also definatley get her into therapy. Let her known your concern for her, that you luv her, etc. Don't use therapy as a punishment or a threat, like "If you don't perk up, I'm making you go to counseling". That will only cause rebellion. Tell her that you want to assist her, and that you think she would probably not want to talk to you, and start interviewing therapists. It also could be depression... which manifests diferently in everyone.

Best of luck!