Cardi B crashed "The Tonight Show Starring Jimmy Fallon" Tuesday night ... just before she revealed her exciting pregnancy news Wednesday morning!

Check out the fun clip ... Cardi hops on FaceTime with Jimmy to promote her new album, cheekily asking if he can see her butt on the cover. When Jimmy says he can "kind of see it," the "Drip" hitmaker tells him he needs to get a better look ... and surprises the audience by walking onto the stage from behind the curtain. Play video content

Cardi has been all about promoting her upcoming project -- titled "Am I the Drama?" -- even taking to the streets of NYC to twerk on a car in front of hundreds of cheering fans at a pop-up event over the weekend.

She promoted her album on "CBS Mornings" Wednesday as well -- while also revealing her

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