Taylor Swift Secretly Wrote "This is What You Came For' and Calvin "Disrespected" Her

If you've been jamming out to Calvin Harris' "This Is What You Came For" featuring Rihanna, well, you can thank Taylor Swift!

But now, Swift reportedly feels disrespected by the DJ/producer. A source tells ET that "during their relationship, Taylor wrote the song, sat down at a piano and did a demo into her iPhone."

The Nashville native sent the song to Harris, who apparently loved it. "They both went into a studio and did a full demo with Taylor on vocals and Calvin doing the beat," the source added. "They both knew the song would be a hit, but Taylor wrote it for Calvin and [they] both agreed it was a bad idea to let the world know they collaborated as a couple -- it would overshadow the song."

Swift has since moved on with actor Tom Hiddleston, and the couple has been keeping their whirlwind romance alive by traveling the world.