The pair wed in a secret ceremony in New Orleans in September and afterwards posted a picture of the happy event on Twitter.
But the singer is said to have finished the relationship at the beginning of December after realising that he wasn't right for her.
According to today's Sunday People, Katie spent Christmas in the UK with her family before jetting back to Los Angeles - on her own - to work on an album with her band, Red Velvet.
Before the wedding, Katie and Brad were only dating for 16 weeks, and she's taken off her wedding ring already.
A source told the newspaper: "Katie did fall head over heals in love with Brad, but it was all too rushed. It soon became clear that it wasn't going to work and she got fed up with him.
"They got married in a very intimate magical ceremony in September but, by November, Katie had had enough. It was a very brave thing for her to do but she's a strong woman who knows what she wants - and it wasn't Brad."
At the time, a loved-up Katie tweeted: "I have never smiled so much in my life! So proud, so happy, so in love, so excited!!"
Katie shot to fame during the X Factor in 2010 when she made it to the live shows under the mentor of Cheryl Cole, but was voted out during week eight.
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