The 37-year-old singer-songwriter recently rekindled his relationship with the 'Dark Horse' hitmaker, but started sending her flowers on a regular basis shortly after she ended their relationship last February.
An insider said: ''She broke it off last year, but John has never stopped trying to get her back - flowers, phone calls, the works. And I guess it finally worked.''
While the brunette beauty, 30, casually dated DJ Diplo for much of last year, John made sure he wasn't spotted on dates with any other women in a bid to prove to Katy that he wasn't over her.
The source told the US edition of OK! magazine: ''Katy ended it just so John would see what he was missing. But now that he's proven himself, she's willing to give the relationship another shot.''
But friends of the multi-million selling artist are concerned she is jumping back into things too quickly with John.
Another source previously said: ''Katy and John are back together.
''Although her friends have warned her to be careful, she is already ignoring all of the red flags.
''She is not listening to anyone and is going into this full force.''
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