(TMZ) - 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. 

TMZ spoke to multiple sources connected to Karrueche Tran, who has said she 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. 

TMZ reports that 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. 

According to TMZ, sources say Karrueche told friends she truly loved Chris and felt that the relationship was great until Rihanna let Brown back into her life.