Miley, 20, and 'The Hunger Games' actor Liam Hemsworth recently called off their engagement and Selena, 20, is reportedly worried her on/off boyfriend Justin will try to hook up with Miley, who he reportedly had a fling with a few years ago.
A source told HollywoodLife.com: ''Justin and Miley hooked up a few years ago. Then he started dating Selena. It created this whole jealously thing between them. Now Selena doesn't want Justin anywhere near Miley -- and they've been fighting about it!''
Liam, 23, and Miley are still living together at their Los Angeles home but in separate rooms after Liam told the singer their relationship was finished for good.
A source told Us Weekly: ''They are definitely over.''
Liam and Miley first met on the set of their film 'The Last Song' in 2009 and had been together four years, with the Australian hunk popping the question last June.
But their romance hit the rocks in February amidst reports Miley had become jealous of Liam's close relationship with actress January Jones.
Liam had been urged by his brother, 'Thor' star Chris Hemsworth, to dump Miley because they thought the relationship was unhealthy for him because she was so jealous.
The heartthrob eventually made the decision to end things after deciding he couldn't cope with the constant drama in his romance with the former 'Hannah Montana' star.