Tarek El Moussa Flips Over His Ex Christina's New Gig!

RadarOnline.com has learned Flip or Flop star Tarek El Moussa isn't exactly happy that his ex-wife Christina recently took a job at Christie's International Real Estate, a direct competitor of his employer, Sotheby's.

"He's been undercutting Christina by telling people he taught her everything she knows about selling houses," revealed an insider, noting the duo both worked as realtors prior to landing their hit show.

Now Christina is "fuming" about her ex's efforts to bad mouth her, and she's fighting back !

"She says she's been a realtor ever since college and that he's the one riding on her coattails," said the insider. "It's getting nasty!"

The tense situation's also causing the pair's already dicey rapport to suffer on the set of their HGTV series.

Tarek, 36, and Christina, 34, "are smiling for the cameras because that's their job, but they're shooting dirty looks or ignoring each other off camera," noted the insider. "To say being around them is uncomfortable would be an understatement."