Laura McDonald posted a 4 1/2-minute video of herself impersonating Stewart Saturday, the New York Daily News reported.
As of Tuesday afternoon, it had gotten more than 700,000 hits.
"Hi Rob, it's me, KStew," she says in the video. "I want to explain what I did. You know, the whole having sex with my director thingy. I'm putting this video on YouTube because I want just you to see it."
Stewart and her married "Snow White and the Huntsman" director Rupert Sanders publicly admitted last month they had an affair and apologized to their respective romantic partners for their behavior.
Stewart and Pattinson, who dated for about five years, are the stars of the "Twilight" film franchise.