The US popstar, who has gained a reputation for writing about past break-ups, dated the One Direction singer for a few months at the end of 2012.
A source close to Swift told The Sun that she is writing about the "very strong feelings" from their short pairing.
The insider said: "Taylor writes music in the same way that other women chat to their friends on the phone. It's been how she deals with her emotions for most of her life.
"Harry and Taylor only dated for a short time but there were very strong feelings, so it's been a tough comedown."
They continued: "Lyrics have been written, but Taylor hasn't come close to deciding whether she'll ever release them. A song about it will surface at some point but it won't be anything like the way she had a dig at her other ex John Mayer in one of her other tracks."
Yesterday, Swift confirmed that she had begun recording again, tweeting: "Back in the studio. Uh oh..."
Various reports have surfaced over the cause of the break-up, which is said to have come after a furious argument while on holiday in the British Virgin Islands.
Swift is alleged to have told Styles that he was "lucky" to be with her, while Styles was reportedly frustrated that she "didn't want to put out".