The 'Heart Attack' hitmaker, who previously admitted her long-time friendship with the 'Spring Breakers' actress, 21, has been strained in the past, sent the rumour mill into overdrive when she stopped following Selena on Twitter on Wednesday (02.07.14) shortly after posting a cryptic angry message.
The 21-year-old singer allegedly wrote: ''Swimming away from the bullsh*t bye b*tch (sic),'' but the post has since been deleted.
The duo, who have both sought treatment in rehab, have been friends since they starred in 'Barney & Friends' as children.
A source also told gossip website HollywoodLife.com: ''Demi doesn't need any extra drama, she's learned from her own recovery to lose the people that bring her down and Selena is to much of a headache.
''The unfollowing of Selena is sending a message that Demi is p***ed. She is sick of Selena's boy-who-cried-wolf mentality.''
The 'Neon Lights' hitmaker and Selena, who is also said to have distanced herself from a number of her other celebrity pals, including Taylor Swift, Kendall Jenner and Kylie Jenner, have had a complicated relationship in recent years.
Last year, the former 'X Factor' USA judge said: ''We've been through a lot together, we've had periods of times we here we grew apart and we just didn't really talk.''
But she added: ''Now we are in a place where I think we realised that life is so short and that when you have people in your life that love you so much then, you should just always be around them.
''Everything about her, she has a great soul, she's talented, she's beautiful, and she will always be really, really close to my heart.''
'The Now Now' will be released on June 29th, and two new teaser tracks have appeared - 'Humility' and 'Lake Zurich'.