"She was furious and felt like the producers were ganging up on her," says one insider. "She literally lost her mind."
Singer, 53, was shooting with Sonja Morgan and Alex McCord when she lost it, another source adds.
"No one is really sure what exactly spurred her craziness," the second source tells Us. "But she got herself worked up over filming this scene and one thing led to another, and she punched the producer. Literally."
Adds the source, "They were supposed to be filming all day at Ramona's apartment, but obviously that ended after the fight." With only one month into shooting, the first source says producers are trying "to mediate the situation so the cast all stays filming."