The actress said it was 'weird' filming the scene with co-star and rumoured boyfriend Robert Pattinson.
"There are two big scenes in the two films and we did them fairly early," Stewart told Glamour Magazine.
"It was so weird, it didn't even feel like we were doing a scene within a Twilight film. I was like, 'Bella! What are you doing? Wow! What is happening here?' It was very surreal."
The actress also revealed the film was originally given an R rating because of the explicit sex scene.
It was confirmed earlier today that the scene in question had been cut down in order ensure the film received a 12A rating from the BBFC.
Twilight: Breaking Dawn - Part 1 will be released in cinemas on November 18.
> Kristen Stewart: 'I'm not bothered by rumour'
Watch a new clip from Twilight: Breaking Dawn - Part 1 below: