GEORGE CLOONEY’s GF STACY KEIBLER  hit the roof when she learned that he may have had a secret affair with  “Real Housewives of New York City” star Carole Radziwill, say insiders.

“Stacy was fit to be tied,” revealed a source close to the leggy beauty. “She read George the riot act.

“Stacy knows he’s hooked up with a lot of other women before they started going out. But she thought she knew about all of them. Now Stacy is wondering who else George didn’t tell her about.”

The trouble began when Radziwill’s “Housewives” co-star Sonja Morgan dropped the bombshell on the Aug. 27 episode of the Bravo reality show.

While watching Radziwill on the beach, Morgan nonchalantly remarked: “Look at the ass on Carole. No wonder Clooney did her for a year.”

Neither Morgan nor Radziwill, 49, offered any additional details about the reputed relationship with the 51-year-old actor.

But Radziwill tweeted: “I don’t kiss and tell, but if I did, I’d say Clooney was a very good kisser…”

Those teasing comments “just inflamed Stacy even more,” said the source.

George reassured his stunning girlfriend that he never had a relationship with Radziwill, that it was nonsense designed to hype the show, according to the source.

Even after that explanation, the 32-year-old blonde beauty grilled her beau mercilessly, and he kept insisting there was “nothing to it.”

But Stacy, a former professional wrestler, “can be extremely jealous,” said an insider. “She wants George to put all his cards on the table – about everyone!”