JENNIFER ANISTON  unleashed her fury by calling JUSTIN THEROUX a cheater after learning he was still carrying on with his ex-girlfriend Heidi Bivens, say sources.

Jen, 43, exploded when she discovered that her beau was secretly communicating with Heidi, the beautiful costume designer he’d been dating for 14 years before dumping her to hook up with Jen.

“In Jennifer’s book, what Justin did was cheating – he was going behind her back to maintain ties with his ex,” revealed a close source.

“She was furious and read him the riot act, plunging their relationship into crisis.”

For months, Justin, 40, had been “calling, texting and e-mailing Heidi without Jen’s knowledge,” confirmed another insider.

“They stayed in touch very sporadically at first, but now they’re talking all the time. He’ll e-mail, text or call – when Jen’s not around, of course – and they talk about everything. He asks Heidi’s advice about his career and even discusses his love life with her.

“For Jen, it’s the ultimate betrayal. Justin has a lot of explaining to do to regain her trust, and it may be too late already.”

Jennifer’s rep denies the couple is stressed over Heidi, but as The ENQUIRER reported on Jan. 20, witnesses saw the pair bickering in public at Hollywood’s trendy Tower Bar.