Forget performing at the Super Bowl, Beyoncé was nervous about showing her new HBO doc, “Life Is But a Dream,” last night at the Ziegfeld Theatre. The singer — who stars in, produced and co-directed the film — told Oprah Winfrey that she woke up yesterday morning and “thought, ‘What did I do?’ ” “I’ve been anticipating this moment . . . this is huge for me . . . and I felt that it was time for people to see who I am. This is three years in the making.” She said that the film concludes with the birth of her daughter, Blue Ivy. “Once I saw my daughter, I knew that the end to this story, this documentary, was here and I was ready to tell the story,” said the singer. Oprah said that she had recently watched the film with nine students from her Oprah Winfrey Leadership Academy for Girls in South Africa. “They said, ‘Mama O., she’s just like us. And I said, ‘Meh, kind of, kind of not. Let’s not forget she is Beyoncé,” Winfrey cracked. Bey also revealed that she got approval from her mother, Tina Knowles, and hubby Jay-Z to shoot the film of her life. “After I gave birth, I felt like a new woman,” she said at the premiere. “I felt like mystique is something extremely important to me, but at some point it’s time, and I’m ready.”