Justin's Instagram war with Selena comes just two months after he re-followed her on the photo-sharing app.
The 'Sorry' hitmaker hit the follow button on...
Justin Bieber un-followed Selena on Instagram after they split up in November 2014.
And they showed signs they were getting on when Bieber re-followed her...
Before Justin re-followed Selena, their only interaction was when the 22-year-old singer shared a throwback photo of them locking lips back in March.
The 'Cruel'...
And way before the pair started feuding Selena admitted that she was tired of answering questions about her former flame.
She said: 'I'm so exhausted....