Have you ever encountered a co-worker who just gets on your last nerve? Cameron Diaz has, and that co-worker just happens to be none other than 'On the Floor' hitmaker Jennifer Lopez.

Diaz is working with Lopez on the film 'What to Expect When You're Expecting,' and according to a source who spoke to Now magazine, Diaz thinks Lopez is a real pain in the you-know-what. According to the insider, "Cam thinks J. Lo's a nightmare and gives movie stars a bad name." The unnamed person then went on to say, "She's even said that Jen should stick to her day job, meaning 'American Idol' and singing."

Them's fightin' words, Cameron! We have to comment on the fact that J. Lo has already starred in several films up to this point, including the epic, heart-wrenching biopic 'Selena.' So, saying Lopez should "stick to her day job" is really a futile statement.

The source goes on to claim that "Jen's constantly taking mini-breaks from the set to see her twins." Apparently, Lopez is a bit high-maintenance when it comes to meal time, too. "She demands to eat at specific times, no matter what," the source tells Now. "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."

Overall, Lopez's behavior on the set of the movie, which is being filmed in Atlanta, Ga., is "driving Cameron crazy." Even though they have their differences, we hope that this doesn't affect J. Lo and Diaz's chemistry on camera. Both women are pros, so we're sure everything will be turn out for the best.