Jennifer Lopez: Something Old, Something New?

Jennifer Lopez is a smart, savvy celeb who knows exactly what she's doing ... so there's gotta be something to the fact that she made a surprise appearance during her ex-husband's concert just days after 86ing her BF.

Jennifer stunned the Radio City Music Hall crowd Saturday night during Marc Anthony's "Private Collection" concert. She sang with him -- in Spanish -- and gave him a big hug when the song ended.

Just two weeks ago, Marc showed up for JLo's Vegas concert.

It's gotta be hard for Casper Smart, who's in NYC trying to resurrect his relationship with JLo.