"During their relationship, Taylor wrote the song, sat down at a piano and did a demo into her iPhone. She sent it to Calvin, who loved it. They both went into a studio and did a full demo with Taylor on vocals and Calvin doing the beat. They both knew the song would be a hit, but Taylor wrote it for Calvin and both agreed it was a bad idea to let the world know they collaborated as a couple ... it would overshadow the song. So Taylor, who kept the publishing rights, used the pseudonym, Nils Sjoberg on the credits."
He released the song and it became and instant smash. Remember... They said they wouldn't collab together. She felt hurt.
"It was a quick downward spiral from that point. One source called it "the breaking point in the relationship." The Met Gala was several days later, when Taylor danced with Tom Hiddleston."