Jennifer Lopez shows up at EX-HUSBAND Marc Anthony's concert #HNNMusic

What's this? 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 he showed up at her Vegas concert too. Must be tough for Casper the ex who is trying to get back with Lopez.

With Files from TMZ