There’s a shocking secret behind Ethan Hawke’s recent affair with a model.

The ENQUIRER has learned exclusively that Ethan cheated on wife Uma Thurman because he believed she had cheated first!

Ethan became convinced that 33-year-old Uma had an extramarital affair with director Quentin Tarantino, 40, while they worked together on his film “Kill Bill,” say sources.

In our August 12 issue, The ENQUIRER reported exclusively that Ethan was cheating on Uma with 22-year-old Jen Perzow, a model he met this past summer while shooting the movie “Taking Lives” in Montreal.

“But what the world doesn’t know is that Ethan fooled around with that woman in Canada

AFTER becoming convinced Uma had cheated on him first!” a source close to the 32-year-old actor told The ENQUIRER.

“Ethan spoke to spies on the ‘Kill Bill’ movie set in China.

“He was really ticked off because he always thought Quentin had a thing for his wife. Quentin and Uma are longtime pals and there’s a real chemistry there since they made ‘Pulp Fiction’ together.

“The ‘Kill Bill’ movie shoot was terribly long — nine months, and a lot of it was shot in China. There were long grueling hours.

“When people around the movie tattled to Ethan, he was really upset. And whether he was seeking revenge or just being completely distraught, he did turn to this beautiful model he met while shooting his movie in Canada.

“He feels bad about how things have turned out. The marriage couldn’t have been very strong to begin with.”

“Now Quentin is worried that he’s going to get dragged into this nasty mess. He’s tried to hook up with some of his old girlfriends since the Hawke cheating story has broken. He doesn’t want to appear unattached because he doesn’t want to be seen as playing a role in this marriage breakup.

“But he and Uma couldn’t hide their affection for each other at the Hollywood premiere of ‘Kill Bill.’ Quentin is like a little boy when he’s around her — he’s totally smitten.”

A published report said that Ethan checked into New York’s Chelsea Hotel on September 13, after complaining to insiders that Uma had booted him out of the house where they lived with their two children in Greenwich Village.

On the morning of September 28, The ENQUIRER caught up with Ethan outside the hotel.

When asked about the present state of his marriage, Ethan said he was only maintaining an office in the hotel, and added: “I don’t really want to talk about anything right now.”