Rob Kardashian's so-called new girlfriend is playing Internet hoax victim ... but we're told she's the one fueling the rumors.

Rob had a new girl, according to some recent online posts -- 'Basketball Wives' and "Bad Girls Club" vet Mehgan James. But it didn't ring true almost from the get-go ... Rob began posting pics of baby mama Blac Chyna, tweeting that he'd never even met James, much less banged her.

Mehgan then put it out there she had no idea how the rumors started ... re-tweeting Rob's denial.

Now the truth ... Mehgan was the mastermind of the whole thing.

Our sources say Mehgan's "team" contacted a bunch of media outlets planting the story. We're told her goal was SOP ... spread her name, grow her following.

Mehgan's at 806k Instagram followers -- doubling in a week -- so sadly, mission accomplished. 

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