Chris Brown's ex-girlfriend ain't buyin' that Chris is "just friends" with Rihanna ... telling her peeps she's convinced they were bangin' behind her back.
We spoke to multiple sources connected to Karrueche Tran ... who tell us KT feels betrayed by Chris because he had promised her that she was the only woman in his life ... especially when she probed him about his feelings toward Rihanna.
We're told Karrueche had suspected something was up between the two of them for weeks because Chris was acting differently -- going to clubs alone, acting distant and ramping up his public appearances with Rihanna.
Sources say Karrueche told friends she truly loved Chris ... and felt that the relationship was great until Rihanna let Brown back into her life.
We reached out to Karrueche for comment -- so far, no word back.