The "New York Post" claims that JENNIFER LOPEZ hooked up with her ex-husband MARC ANTHONY during a recent trip to the city.  She'd left boyfriend Casper Smart back in L.A.

Supposedly, they spent more than two and a half hours together in his hotel.  Marc was the first one to emerge, "He came out really quickly, with his head down, like he didn't want to be seen."

J-Lo came out later, disheveled, but with, "a smug grin on her face."  (You can see what she was wearing that day here.  Definitely not a choirgirl outfit.)