Looks like Gwen Stefani is getting an early start on her Las Vegas residency!

After the Academy of Country Music Awards, Blake Shelton performed a surprise pop-up concert and his girlfriend couldn’t help but get in on the act.

On Sunday night, Shelton tweeted that he was going to perform a free show at the House of Blues in Vegas after the ACMs and was bringing along some “friends.”

And he certainly delivered with Kelly Clarkson and Stefani joining him.

His lady love took the stage around 1:30 a.m. Monday and performing several songs, including “Hollaback Girl” and “Hella Good.”

“The crowd was going crazy for Gwen,” a concertgoer tells PEOPLE.

Stefani wore jeans, boots and a green jacket while letting her blonde hair loose for the night as she led the crowd into a soaring rendition of “Hollaback Girl.”

But boyfriend Shelton had a special request, walking onto the stage – much to the excitement of the crowd – and asked her to sing “Hella Good”.

After a bit of a convincing, the songstress relented – if he agreed to sing it with…