Lambert was there to support East, 27, during his show at Pearl Street Nightclub in Northampton, Massachusetts on Sunday and an eyewitness at the concert told ET that the 32-year-old country crooner kept a low profile up until she hit the stage.
Lambert attended the concert with former Voice contestant, Gwen Sebastian, and another friend that ET's source says was her manager. She was seated in the back with the sound guy for most of the show, but after two songs, East brought up Lambert and Sebastian to perform Mariah Carey's 1995 hit, "Always Be My Baby."
"She was having a blast just singing along and laughing and smiling," the eyewitness recalled. "At one point during the song, Anderson leaned in and gave her a kiss on the cheek and then she gave him one too."
ET's source says after their sweet smooches, East gushed, "I think you all can see what I got for Valentine's Day."
Lambert Instagrammed a pic of the performance, quoting Carey's lyrics in the caption. "'Boy don't you know you can't escape me, ooh darling cause you'll always be my baby,'" she wrote. "I got to spend heart day with great people playing great music. Thanks @andersoneast and your sweet band for letting me and @gwensebastian sing some Mariah with y'all! #beerandmicrophones #valentines #massachusettsisfreezing #werewiththeband"
After the show, Lambert was seen going backstage while East posed for photos with some fans. Once the meet-and-greet was over, she walked out and waved to the concertgoers before leaving out a side exit.
"She was very classy and you could tell she didn't want to take away the attention that was on Anderson," the eyewitness said. "She was there to support her new man."
Lambert started dating East late last year, and a source told ET that the musician "couldn't be more different" from her ex-husband, Blake Shelton. "It's fairly new," the source said in December. "They are just having fun and seeing where it goes."