The Worst Celebrity Feuds

There's definitely some "Bad Blood" here.

Calvin Harris laid into Taylor Swift on Wednesday morning, after TMZ reported that she was actually the one who wrote his hit song "This Is What You Came For."

His tweets started out complimentary at first ... before taking a turn.

"And she sings on a little bit of it too," he revealed, adding, "Amazing lyric writer and she smashed it as usual. I wrote the music, produced the song, arranged it and cut the vocals though. And initially she wanted it kept secret, hence the pseudonym."

Here's where it went awry.

"Hurtful to me at this point that her and her team would go so far out of their way to try and make ME look bad at this stage though," he then tweeted. It's unclear how it's hurtful that her rep confirmed she wrote the song.

"I figure if you're happy in your new relationship you should focus on that instead of trying to tear your ex bf down for something to do," he continued, referencing her new PDA tour around the globe with Tom Hiddleston. "I know you're off tour and you need someone new to try and bury like Katy [Perry] ETC but I'm not that guy, sorry. I won't allow it."

"Please focus on the positive aspects of YOUR life because you've earned a great one," he wrote in conclusion, "God bless everyone have a beautiful day."

Are you team Taylor or Calvin here? Sound off below!