Taylor Swift and Calvin Harris may have learned the hard way not to mix business with pleasure. The pair are said to have ended their one-year relationship in June, though the exact reason for their split was unknown — until now.

According to TMZ, Swift and Harris worked together on his latest single “This Is What You Came For.” The “Out of the Woods” singer reportedly sent her beau a demo of the song with her on vocals, but both decided it was best to keep quiet the fact that she’d written the song. They feared chatter about them having worked together would overshadow the greatness of the song, so Swift, 26, was credited under the name Nils Sjoberg.

Harris eventually recorded “This Is What You Came For” with Rihanna and it took off, topping both the Dance/Electronic charts and Billboard 100 chart. Things changed when the “Feel So Close” DJ sat down with Ryan Seacrest for an interview in which he said he couldn’t see a collaboration with his then-girlfriend working out — in fact, he told Seacrest they hadn’t “even spoken about it.” This reportedly offended Swift so much that one source called it “the breaking point in the relationship.”

Not long after their split, the award-winning musician was spotted cozying up to actor Tom Hiddleston. In the weeks since their first sighting they have been photographed together on lavish vacations, meeting the parents and engaging in some seriously sweet PDA. While many fans have quickly become obsessed with Hiddleswift, others are questioning just how legitimate their relationship really is. It has been suggested that the pair may be engaged in a PR relationship, which would garner media attention for both of them. Several celebrities including “The Big Bang Theory” star Kaley Cuoco, Tom Cruise, Kim Kardashian and Blake Shelton have been accused of participating in similar fake relationships.

Real or not, the picture perfect relationship between Swift and Hiddleston is said to be taking a toll on Harris. On Monday, Page Six reported that the Scottish musician was “not happy” that his ex had moved on so quickly. A source told the magazine that Harris, 32, was “hurt” by her and Hiddleston’s continued public displays of affection, adding that he and his friends believed she was doing it on purpose.

“Looking back, some friends do feel like Taylor’s team manipulated and orchestrated part of [her] relationship [with Calvin] after seeing how public she is with her new guy,” the source said.

Harris’ hurt feelings haven’t stopped Swift and Hiddleston from getting serious. E! News reported that the pair were already discussing taking their relationship to the next level, with one source saying that the “Thor” actor “is the kind of man she would want to marry.” The new couple is also said to have begun discussing having children.

“She has said he would make a great dad. They have talks about what they want in the future and kids are something that they would both like down the line,” the source told E!. “They have very real and serious conversations about life.”