Kevin Costner Stuntwoman Claims She Was Menstruating During Alleged Unscripted Rape Scene

12:38 PM PT -- Costner's attorney Marty Singer is firing back, telling us ... “This may be a new filing, but it’s really just more of the same old nonsense. These claims are not only groundless, they are ridiculous. There are 12 independent witnesses who confirm that Ms. LaBella’s claims are fabricated. Kevin Costner has been very clear about why he refused to give in to a transparent shakedown. He is determined to stand up and fight for the truth - and we will continue to do so.”

The stunt performer suing Kevin Costner over an alleged unscripted rape scene on a movie set is adding some new details to her lawsuit ... she says she was menstruating and her underwear was exposed when her pantaloons were ripped open.

Devyn LaBella is suing Kevin for sexual harassment, discrimination and retaliation for an alleged incident during "Horizon 2" filming ... he's trying to get her lawsuit dismissed, claiming the woman is mischaracterizing what went down on set ... but now she's firing back.

In new legal docs, obtained by TMZ, LaBella claims during the alleged unscripted rape scene her pantaloons were ripped open from the waist down to the crotch ... making her panties visible at a time when she was on her period.

LaBella says she had no issue filming a scripted rape scene the day before ... but she says when she came to work the next day, Kevin wanted the cast and crew to work out an unscripted rape scene.

She claims she was "trapped in the tight confines of a covered wagon" as her character was being raped by a character named Birke, who was played by actor Roger Ivens.

This time, LaBella claims Kevin ignored the requirement to have an intimacy coordinator present for the scene ... and she says she was pinned by Ivens' large, muscular body. She says he put "his hand on or near [her] breast and vaginal areas" and stretched his leg and buttocks across her body, sometimes getting on top of her on all fours and mounting her.

For his part, Kevin's claimed LaBella's pantaloons and petticoats underneath remained undisturbed during the scene.

As we first reported ... Kevin filed docs last month to dismiss LaBella's suit, calling her accusations "absolutely false" and saying he found it "deeply disappointing" that someone who worked on the production would try to damage his reputation and embarrass him.