A source close to the rapper tells us ... Sexyy Red's heading into the new year single -- so, she's finally got time for handling all of her personal care -- and, that includes a breast augmentation.

ICYMI ... SR posted a pic to her IG Story a few hours ago with the caption, "Didn't want small no more I'm goin back big."

Our sources say she's not going huge with her new chest ... she's undergoing surgery because her breasts are causing way too many problems -- including back pain and ruining the fit of her clothes -- giving her an unflattering, disproportionate appearance. So this is more about redistributing things.

Sexyy Red has no qualms about taking off her top, BTW ... recently baring all in an Instagram Live video -- leaving a lot of fans with hearts in their eyes.

We're told this is Sexyy's third surgery ... and, she feels like the third time's the charm.

BTW ... Sexyy's Northside Princess makeup line is doing really well, we're told -- and, our sources say she plans to enjoy some personal time before returning to music with new energy -- a move straight out of the Rihanna playbook.