Karrueche Tran & Chris Brown Break Up -- Rihanna Was The Last Straw

Chris Brown played the Rihanna card one too many times with Karrueche Tran ... we've learned Karrueche has not only broken up with him ... for safe measure she changed her number.

Our sources say Karrueche exploded when Chris recently "liked" 2 fan photos on Instagram of Rihanna and him together.

But Rihanna was only a small part of Karrueche's problems with Chris. We're told she was unhappy for a long time ... as sources put it, because "Chris acts immature, like a child." She thinks he's ruining his life by spending more time partying than making music.

Chris' reaction to the break up has been breathtaking. He went to St. Tropez this week and took a very good friend of Karrueche's along for the ride. And he made a point to be photographed with her ... making sure Karrueche would see it.

Karrueche's reaction was not jealousy. We're told she's changed her phone number ... specifically so Chris couldn't contact her.

Boy ... it takes guts.