Cardi B is out and about, and her trip to a New Jersey gym has us wondering ... is there a new baby boy in her life?

In the short video -- shot Thursday in Tenafly, NJ -- Cardi's engrossed on her phone while sitting on a bench. Eyewitnesses tell us she was taking a break from her workout. Getty

Interestingly .... she was wearing a sports bra and leggings ... not hiding her midsection.

We're told she hit the gym with two trainers -- one lady and one dude -- and she was in grind mode.

It's been awhile since we've seen Cardi B out and about ... with the star staying out of the public eye in the latter stages of her pregnancy. Play video content

As you know ... this would be Cardi's fourth child -- her first with NFL superstar Stefon Diggs , who she started dating shortly af

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