We're told Tran had absolutely no idea her boyfriend was a daddy -- much less a daddy with a baby mama with whom SHE partied.
Our sources say Karrueche and Brown had a blowout fight a week ago and weren't speaking, but he tried calling her after the story broke ... she did not pick up.
DNA tests performed in the last week show conclusively ... Brown is the father of a 9-month-old baby with a former model named Nia. We're told he and Nia aren't romantic but they get along fine, and Chris has made some sort of financial arrangement with her.
As for Karrueche ... we're told she's actually relieved, because it now gives her a clean, permanent out.