Everything was perfect. To celebrate turning 27 on August 10, Kylie Jenner whisked some of her closest friends off to paradise. “Best 𝐛𝐢𝐫𝐭𝐡day ever??” she captioned a carousel post on Instagram of the celebration, which took place on a yacht, complete with giant balloons spelling out her name.

“Kylie was on top of the world,” says an insider. “Nothing could ruin her mood.”

Or so she thought. The moment Kylie returned from the trip, she learned that her boyfriend of 16 months, Timothée Chalamet, had been seeing other women behind her back. “Kylie and Timothée have never said they’re exclusive, so in his book he hasn’t done anything wrong,” says a source close to the pair. (A rep for Timothée denies the story.) “Kylie, of course, feels differently. She’s betrayed — not to mention humiliated.”

Timothee Chalamet Is a Kid in a Candy Store

Timothée simply can’t resist the perks of his skyrocketing fame. “There’s a double-edged sword to his extreme jump in popularity over the past year,” says the source. “It’s put him in a bubble, bloated his entourage, and robbed him of his last bits of anonymity. But it also means that women do not say no to him, ever, and are totally open to no-strings- attached brief encounters. Temptation is everywhere.”

Of course, Kylie has had men come on to her, “but she wouldn’t think of cheating on Timothée,” says the source. “Even though she didn’t want to label their relationship, she thought he was The One. Little did she know he’s a raging player. Women are definitely his vice.”

Timothee Chalamet Stepping Out on Kylie Jenner

Kylie has said very little about their romance. (They made their first public appearance in September, making out during Beyoncé’s Renaissance concert in LA.) In the new British Vogue, Kylie confides that she has made the active decision to keep their relationship away from the spotlight, saying it “feels good. Privacy is so important to me in life.”

For his part, “Timothée has said zilch about Kylie,” adds the source. “He talked about the Beyoncé concert but just about how he’s a big fan of the singer.”

Will Kylie Jenner Give Timothee Chalamet a Second Chance?

What happens to the couple now remains to be seen. “Kylie is angry, but she also knows she and Timothée are a power couple. They both benefit from this relationship,” says the source. “So Kylie will probably give him another chance. Maybe now they will finally have that “exclusive” conversation, so they both know where they stand.

“The problem with that,” says the source, “is Timothée is always going be tempted by the ladies.”