A 39-year-old man is said to have repeatedly sent the Xtra Factor co-host gifts, also trying to touch her when he showed up at her work.
He is currently on bail and awaiting further questioning by police next month.
"It's been a pretty stressful past few weeks for Caroline," a source told The Mirror.
"But now the police are involved, she hopes the matter will soon be resolved and she can move on with her life."
A Metropolitan Police spokesperson said: "A 39-year-old man was arrested and questioned, and has been bailed until mid-August pending further investigation."
The man reportedly started turning up at X Factor auditions in Manchester, and followed her Twitter updates to track where she would be based as auditions continued.
One Direction star Harry Styles is said to have recently said to be keen to rekindle his romance with former lover Flack.