(Cover) - EN Showbiz - Cameron Diaz thinks co-star Jennifer Lopez is a "nightmare" on set.

The pair are currently filming comedy drama What To Expect When You're Expecting together. According to an insider, Cameron is not happy with Jennifer's unprofessional behaviour.

"Cam thinks J-Lo's a nightmare and gives movie stars a bad name," a source on set in Atlanta, Georgia told Now magazine.

"She's even said that Jen should stick to her day job on American Idol."

Cameron finds Jennifer's time management disruptive. The numerous breaks that Jennifer take while they are shooting are a major annoyance for Cameron.

"Jen's constantly taking mini-breaks from the set to see her twins," the source explained. "She demands to eat at specific times, no matter what.

"So at 10:15am on the dot, she stops work and has an assistant run over to her with a small protein and veg-based meal. It's driving Cameron crazy."

What To Expect When You're Expecting stars Cameron, Jennifer, Elizabeth Banks, Chris Rock and Joe Manganiello and will be released in 2012. (C) Cover Media