We have more info on Carrie Prejean's abrupt about-face during the settlement negotiations with Miss California USA lawyers -- when they showed her the X-Rated version of Home Alone ... starring Carrie herself.

When the video started playing, Carrie's first reaction was "that's disgusting" ... and Carrie denied it was her.

Then, the camera angle changed ... and panned up to her face. She was caught red-handed ... so to speak.

Carrie was rendered speechless and immediately began talking with her lawyer. We're told it took about 15 seconds for Carrie to drop her $1 million dollar demand.