Taylor Swift Is Swinging Back At Calvin Harris: Get it Straight ...

Taylor Swift is swinging back ... calling BS on Calvin Harris' Twitter tirade in which he claims he was the architect behind the hit song, "This Is What You Came For."

Sources close to Taylor tell TMZ ... Calvin's intentionally downplaying her involvement, essentially saying he did all the music. 

But Taylor says not only did she write the lyrics ... she also created the melody, and those 2 elements are the essence of any song. 

Calvin Harris Suspected for Weeks Before Breakup, As He feels Betrayed Over Taylor's New Hookup

Calvin Harris suspected something was up with Taylor Swift for weeks before their breakup ... and his suspicions were confirmed Wednesday when pics surfaced of Taylor making out with Tom Hiddleston ... this according to sources connected to the former couple.

Our sources say Taylor and Calvin were in a "committed relationship," but he suspected something was up in early May ... a month before their breakup.

As we reported, at the time of the breakup Calvin posted there was still "a huge amount of love and respect."