Though “Red” has yet to run its course, Taylor Swift is already thinking about her fifth studio album.

“It’s starting, the anxiety is starting, and when the anxiety starts, the writing happens right afterwards usually,” she said backstage Thursday night at CMA Music Fest in Nashville shortly before she took the stage at LP Field.

“I like to write for two years before I finish an album,” she continued. “Whatever I write the first year is going to get thrown away because I’m going to sound [like what I’ve done before.]”

As she rounds the second year of writing, “it ends up sounding like the next subject,” she says. “I feel like staying the same is the easy way to go, but it’s not where I want to go musically.” Though one would think a call to write with Swift would be one most songwriters would be happy to get, Swift added, “it’s harder to call up people you don’t know and write with them for the first time.”

Swift, who released a video for her duet with Ed Sheeran, “Everything Has Changed” Thursday, called the clip “one of my favorite videos that I’ve ever been a part of because the  kids were so good.”  The sweet clip features mini-me,  7-year old versions of Swift and Sheeran.

Not surprisingly, the singer was still on a high from her performance with the Rolling Stones Monday night in Chicago. “It was everything you think it would be but better,” she said. “They handwrite notes thanking you for being there. You get pictures with them.” As far as her and Mick Jagger’s onstage waltz, Swift said that came from their working through “As Tears Go By.” “The musical director and Mick and I were rehearsing the song with a piano and we started dancing a waltz with overly exaggerated ballet moves. We forgot the words and we incorporated a twirl into the performance in homage to our hilarious rehearsal.”