J Lo took over the dance floor at Ben's Georgia estate during their wedding celebration, grabbing the mic and performing a new song while Ben sat alone in a chair ... just a few steps away from the new Mrs. Affleck.

The lyrics are pretty affectionate ... Jen's singing about passion, still being in love and there's even a catchy hook, "Can't get enough."

J Lo's doing it all in her wedding dress, and she's even got backup singers and dancers!!!

For his part, Ben really seemed to enjoy his front row seat ... just check out the loving look on his face as he watches his bride strut her stuff.

As you know ... Ben and Jen's huge ceremony and party went down last weekend -- at the same venue where they were supposed to get married 20 years ago -- with tons of friends and family there to celebrate the newlyweds.