Here's how it all went down ... before Nicki got on stage at London's Wireless Festival, she went Live on Instagram for a little pre-show chat with the Barbz. When one of them asked if she and Kenneth Petty were expecting their 2nd baby, she immediately responded, "I'm not fat, guys, I'm pregnant. Sorry" ... and then she rolled her eyes.

Of course, her screen was immediately flooded with congratulations and that's when Nicki realized ... "Oh wait. Did I say it wrong? I'm sorry. I think I said it wrong. I meant to say, I'm not pregnant. I'm fat."

Hmmm ... we're thinking Nicki's too social media savvy for that kinda screw-up. We smell a troll job, but a funny one. Nicki and her husband had their first kid, a baby boy, back in September 2020 -- and they still haven't revealed his name.

As we reported, Kenneth is gonna have a lot of face time with the kid, as he was sentenced to one year of home confinement -- in their luxurious Calabasas mansion -- for failing to register in California as a sex offender.