LAS VEGAS, NV - MAY 17:  Recording artists Taylor Swift (L) and Calvin Harris attend the 2015 Billboard Music Awards at MGM G
Kevin Mazur/BMA2015/WireImage

Update 12PM ET: Taylor Swift's rep confirmed that she co-wrote Calvin Harris and Rihanna's "This Is What You Came For."

Original: Swifties and non-Swifties, take this with a grain of salt, but there's yet another theory about why Taylor Swift and Calvin Harris, together for 15 months, broke up. According to TMZ, Swift is mad because she secretly wrote Harris and Rihanna's "This Is What You Came For," but Harris was weird about saying if he'd collaborate with his girlfriend in interviews.

Yeah, juicy, right? Apparently (*uses hushed gossipy voice*), Taylor cut a demo of the song with a piano and her iPhone and sent it to Calvin, who then produced the song into something much larger. They both agreed that being on a song together wasn't the best career move (keep in mind, the last time Swift collab'd with a boyfriend was on John Mayer's "Half of My Heart" in 2009, and it will go down in history). So she retained credit for "This Is What You Came For" under a pen name, Nils Sjoberg.

But when Harris (born Adam Wiles) was promoting the track, Ryan Seacrest asked if he'd work with Swift. "You know, we haven't even spoken about it," Harris replied. "I can't see it happening though."

This response, according to TMZ's sources, had Swift miffed. And that's the rumor.

The idea that Swift was a cowriter on "This Is What You Came For" has been floating around the fandom for a minute. Swift's name had appeared on the song's Wikipedia page, only to be replaced by Nils Sjoberg. Seventeen did some reporting and found that Nils Sjoberg is probably not a songwriter.

The Cut is calling this theory the most believable yet, ranking it higher than the rumor that Swift cheated on Harris with current boyfriend Tom Hiddleston. But, like I said, a tiny grain of salt...

Next, watch Calvin Harris talk about his collaborative process: