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Cardi B‘s wasting no time getting ready for her upcoming tour … just days after having her fourth child, TMZ’s exclusive video shows she’s already back working on her fitness!

We got the clip of her Thursday morning in a New Jersey gym, rocking just a sports bra and leggings as she went through a workout sesh … you can see she wasn’t shy about showing her midsection. The whole scene made us think Cardi might have had the baby, and now that she’s confirmed the birth, it all makes sense.

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Eyewitnesses tell us she hit the gym with a couple trainers, and — other than taking short breaks — she was intensely focused on exercising.

As we reported … Cardi gave birth last week to a brand new bouncing baby boy, her first with NFL superstar Stefon Diggs. No word on the kid’s name yet.

In Cardi’s IG announcement, she makes a clear reference to our gym clip, saying, “I’ve started prepping for tour getting my body right getting my mind right.”

Her “Little Miss Drama” tour launches February in Palm Desert, CA … and, as Cardi put it, she’s planning to give her fans “the performance of a life time!”

Someone’s in prime twerkin’ shape!