Justin Bieber wasn't crying crocodile tears ... he really did get choked up and overcome with emotion when his VMA gig ended, and he's blaming his nerves for the sobfest.
Sources at the award show tell TMZ ... the Biebs was super nervous before his big "What Do You Mean?" performance ... because it's been five years since he last sang at the VMAs.
Although Justin included a message about his "journey" -- with clear religious undertones -- we're told his break down was more about a sense of relief that he got through the comeback performance.
Justin recovered just fine from his butterflies though -- one video posted on Shots showed him onstage at Henry's singing his new hit again. No tears this time.
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