Chris Brown's Girlfriend Karrueche Tran -- Oh, By the Way ... We're Happy Together!

Lost in all the Chris Brown/Rihanna possible reunion madness is the fact that CB has a serious girlfriend -- and sources close to her tell TMZ the two of them are doing quite well in their relationship ... thank you very much!

Sources close to Karrueche Tran tell TMZ she is unfazed by the recent rash of RiRi rumors. In fact, we're told CB and KT are now actually living together.

As for CB showing up to Rihanna's birthday party, we're told that didn't bother Karrueche either ... because he came home to her that night and whisked her to Miami the next day to celebrate Valentine's Day.

According to our sources, Chris professes his love to Karrueche daily and he has no plans to leave her for Rihanna.