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A popular porn star who appeared in the movie "Buttman's Stretch Class #4" is suing the male lead (Buttman) ... claiming he failed to disclose that he was HIV positive before they shot sex scenes.

The woman behind the lawsuit is Katie Summers -- real name Katherine Howard -- who claims she was paid to appear in the film back in 2009 ... and the role involved sexual interaction with John Stagliano (aka Buttman).

In Katie's lawsuit, filed in L.A. County Superior Court and obtained by TMZ, the porn star claims Stagliano had known he was HIV positive since 1997 ... but failed to disclose that information before they shot their sex scenes for the movie.

FYI -- Stagliano has been open about his condition for years.

Still, Katie claims she didn't find out about Stagliano's condition until 2012 ... 3 years after they shot the movie.

In her suit, Summers does not say if she is HIV positive.

Summers is suing for negligence, intentional infliction of emotional distress and sexual battery. She's also suing John's production company -- Evil Angel Productions -- which produced the flick.