Carrie Underwood on Having Kids, Marriage and Her 'Emotional' New Single

                                                                                       (Image Credit: ABC News)

She's had more success than she'd ever dreamed of, but at 29, country sensation Carrie Underwood is grappling with the same question every career-driven woman considers: When is the right time to start a family?

"To be honest, one day I'm absolutely fine and happy and, you know, start legitimately thinking about the idea of having kids, and the other day, I'm like 'oh gosh, no, no, what?' I can barely keep my own schedule straight,"  said Underwood, who married pro hockey player Mike Fisher in 2010.

"Right now in my life, I'm smart enough to know that I'm a selfish person right now, and I can be, and that's OK. There's nothing wrong with that. At least I recognize it."

"That's a big step," she said. "It's like moving away from home. It's really scary until you do it and then you figure it out," the singer added.

In an interview with "Nightline," Underwood talked about family and juggling her rigorous life on tour with her husband's schedule - Fisher plays for the NHL's Nashville Predators.

Despite being apart for long stretches of time, Underwood, who has a hit single called "Before He Cheats," said she never worries about Fisher being faithful.

"I know he loves me and I know he loves God. And he wouldn't do that to either one of us," she said. "Every guy I've ever dated before Mike, somewhere in the back of my head, I would always have trust issues. With Mike, it never crosses my mind."

The five-time Grammy winner has had a soaring career since she first broke into the national country music scene on "American Idol" in 2005. In addition to having several multi-platinum albums and 11 No. 1 hit singles, Underwood just earned five 2012 CMT Music Award nominations, which were announced Monday.

Her new album, "Blown Away," drops on May 1 and it features a love song, "Forever Changed." Underwood said it's about a woman who is married with children and is coming to the end of her life. She said one of the writers wrote the song about his mother who was suffering from Alzheimer's, but Underwood said it reminds her of her grandmother's death.

"I get emotional even talking about this song to be honest. It's just beautiful," Underwood said, tears welling up. "It just becomes very real… and I remember my grandmother at the end of her life, looking at my dad and thinking that that was my grandfather."

"I can never sing this song, like on stage in front of people. I could barely get through it in the studio," she added.

Watch the full interview with Carrie Underwood on " Nightline" TONIGHT at 11:35 p.m. ET/PT

ABC's Lauren Effron contributed to this report.