• Selena Gomez made a surprise appearance at Julia Michaels' concert in Los Angeles on Monday.
• They sang their duet "Anxiety" and ended the performance by kissing onstage.
• Gomez and Michaels are frequent collaborators and recently co-wrote Gomez's first No. 1 single, "Lose You To Love Me."
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Selena Gomez surprised fans at Julia Michaels' concert in Los Angeles on Monday — first by showing up to perform, and then by kissing Michaels onstage.

Gomez and Michaels performed their introspective duet "Anxiety," which was released in January alongside Michaels' EP "Inner Monologue, Pt. 1."

Fan videos quickly circulated of the surprise performance, with many writing that Gomez's live vocals have improved and that she seems happy and healthy.

But the bigger surprise came after the song, when Gomez was getting ready to exit the stage. As the "Bad Liar" singer embraced Michaels, she leaned down and the two shared a quick kiss.

In fact, Gomez and Michaels embraced a number of times throughout the performance. Entertainment Tonight's Farai Bennett caught one of the instances on video just after Gomez walked on stage, as well as a sweet moment just before they began singing.

After their performance Monday night, Gomez posted a Polaroid photo of herself with Michaels on Instagram.

"My love, my heart and my soul," she wrote.

The two musicians are close friends and frequent collaborators. Most recently, they co-wrote Gomez's first No. 1 single, "Lose You To Love Me," an emotional ballad that many took to be about Gomez's ex Justin Bieber. (Interestingly, Michaels also co-wrote Bieber's hit "Sorry," which he has confirmed is about Gomez.)

• Read more:
• Selena Gomez says she 'dodged a bullet' in surprise second song, seemingly about moving on from Justin Bieber
• Selena Gomez confirmed she's been 'super, super single for 2 years' since splitting from Justin Bieber
• Justin Bieber and Selena Gomez's songs were made into a viral mashup and it's giving fans closure
• Selena Gomez opened up about being attacked for her weight following her lupus diagnosis: 'That really messed me up'

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