"I've got to tell you something, it is off the charts right now. I am spread so thin," the singer told PEOPLE Monday night at the Emmy Awards, where she was a presenter.
"When I left today to come here, they were like, 'Oh! Mommy! Ahh!' They're not used to it. They're like, 'Where are you going?!' So I told them I was going to get in their bed when I get home and wake them up and kiss them."
Her three sons with Gavin Rossdale – Kingston, 8, Zuma, 6, and Apollo, 6 months – may have to get used to their mom heading out a little more often now that she's joined NBC's The Voice as a judge.
On that score, Stefani, 44, couldn't be more excited.
"It happened so spontaneously, I had no idea," she said. "I had that baby, little guy, didn't know what I was going to be doing next. And then they called, and I was like, 'Alright, I'll do it.'
"It was just perfect timing for me, and it's just such a different, inspiring thing to be around music in that kind of a way, and wearing a hat where I'm actually helping people. It's very cool. Really good."
Stefani wore an eye-catching Versace gown with tiny Swarovski crystals on Monday. But just as notably, she didn't wear her trademark red lipstick – and she didn't at Sunday's MTV Video Music Awards, either.
"I wore pink last night, that's right," she said. "I don't always wear red lipstick. My husband really likes me in red lipstick, so most of the time I wear red lipstick, but he's not here tonight, so …."
Reporting by JENNIFER GARCIA