"I was nervous, but also excited … I never thought in my wildest dreams she was going to react the way she did," Montag, 23, tells PEOPLE.
The reunion – which was filmed for The Hills – was less than joyous. "She was looking at me almost like a zoo animal. It wasn't like I was her daughter anymore. She was looking at me like I was a circus freak," says Montag.
"I think she was most upset that Spencer was there for me and she wasn't. She was saying how sad it is that my confidence is gone because of the music industry I'm trying to get into."
Montag, who just released her album Superficial, still struggles with daily pain but has no regrets about what she's done. "I wanted to be the most beautiful, inside and out. It's part of being a pop star," she says.
So where does her relationship with mom stand? "I tried to leave it as best as I could, but my heart was breaking."
For more on Heidi's confrontation with her mother, pick up this week's issue of PEOPLE, on newsstands Friday