What is the mental illness of Tiffany in Silver Linings Playbook?

The official word (from the book blurb) is that she's suffering from clinical depression.

When Pat meets the tragically widowed, physically fit, and clinically depressed Tiffany, she offers to act as a liaison between him and his wife, if only he will give up watching football, agree to perform in this year’s Dance Away Depression competition, and promise not to tell anyone about their ‘contract.’ All the while, Pat keeps searching for his silver lining.

As far as her 'undiagnosed' conditions are concerned, the author has suggested (in this interview) that she may also be suffering from anxiety issues:

Could you tell me about the creation of Tiffany? Was the character inspired by anybody you know?

It’s funny; people ask me that all the time, like, “Did you know someone who needed to sleep with everybody in the office?” and the boring answer is “no.” I think what Tiffany really represents is that explosive quality that forces you to take a hard look at yourself. Pat is delusional – especially in the book, he’s much more delusional in the book. He just does not want to face reality and Tiffany is the one that grabs Pat’s head and makes him look squarely in the mirror at someone that he doesn’t want to see.

...

Growing up in a blue-collar neighborhood, you didn’t talk about stuff like that. To use the word “depression,” that was not something I was comfortable with for most of my life and I think my wife really forced me to look at the fact that I was not happy, I was depressed, I had anxiety issues and I wasn’t really living the life that I wanted to live. That was a hard thing for me to admit in my 20’s. I think, metaphorically, that experience probably inspired, on a subconscious level, Tiffany.