Nikki Bella I'm Havin' a Fit Day

6/3/2018 7:32 AM PDT

Nikki Bella Hits the Gym with Her Sister and Her Niece

"Total Divas" stars Nikki and Brie Bella hit the gym Saturday with baby Birdie ... and no sign of Cena.

Nikki kinda sorta back with ex-fiance John Cena.  As we reported, Nikki told TMZ she and John are working on getting back together but the deal isn't sealed ... at least not yet.

It's interesting ... the conflict between the 2 involves a baby.  For a long time, she wanted one but he didn't.  It makes the kid in the pic more relevant.

John has said he has had a change of heart and now wants kids.  Nikki tells us she believes he's being sincere, but she says she's got some thinkin' to do.