WE FINALLY FIGURED IT OUT . . . SERENA WILLIAMS GOT CHEEK IMPLANTS!! (BEFORE AND AFTER PICS)

WONDERING FOR MONTHS what tennis star Serena Williams did to her face. We knew she had SOME WORK DONE . . . but we couldn't FIGURE IT OUT.

But last night at the Miami Heat Game it was obvious . . . she got CHEEK IMPLANTS.

As the AFTER pics below show, Serena's cheeks are SIGNIFICANTLY BIGGER . . . even in photos where she's barely smiling.