The teenage singer controversially dated 32-year-old Flack for a brief period last year, but they eventually parted ways in January.
However, it has been claimed that Styles still has feelings for the Xtra Factor host and has trying to contact her recently.
"Harry has never quite got over what he had with Caroline. They had such fun together," an insider told The Sun.
"He sent her a few texts just to test the water as a lot has happened since they were together - particularly Harry being linked to a series of girls."
The source added that while Styles is keen for a reconciliation, Flack has concerns about a repeat of the backlash that came with the couple's initial fling.
"She's also a touch wary because of the abuse she got last time for dating him," they said.
Styles's romantic life was under scrutiny again recently when it emerged that he had an affair with married radio DJ Lucy Horobin, also 32, last year.