Lindsay Lohan "freaked out" and refused to film scenes in Scary Movie 5.

The 26-year-old actress - who has battled drink and drug problems in the past - allegedly tried to avoid a scene ridiculing her troubles, so she missed her flight to the set in Atlanta on Sunday and went to hospital claiming to be suffering from "walking pneumonia."

"Lindsay missed every meeting she had for the film, including script reads and wardrobe meetings. Then she missed her flight to Atlanta on Sunday to shoot the movie. The producers had been getting signs Friday that she was a mess, and would not be fit to work. She is under contract - so to get out of it, she had to prove that she was sick. She tried to prove she has walking pneumonia," an insider told The New York Post.

Lindsay is thought to be particularly concerned about the horror spoof's script - which features a kiss with co-star Charlie Sheen - because of how much it pokes fun at her.

"It went pretty hard at Lindsay. She'd been freaking out about it for weeks," a source said.

Another insider told the publication, "She's been locked up in her room at the Bowery Hotel. She's been in a tailspin. Even Charlie Sheen worried she might not be able to do the scene."

But a representative for Lindsay claims she has been "willingly" completing all her obligations for filming. "These stories are untrue, and she is willingly fulfilling all of her obligations," they explained.