The 24-year-old singer publicly scolded Justin when he threatened to make his Instagram page private after fans lashed out over pictures he posted with his new girlfriend Sofia Richie, 18, and her pals are said to be worried that it is a sign she is "over-worked and under pressure".
A source told The Sun: "Justin and Selena's relationship is notoriously rocky and it hasn't been easy to see him move on so publicly with Sofia.
"But Selena is used to seeing him with other women - it's the combination of being over-worked and under pressure.
"She's been touring for four months non-stop and won't rest until mid-December with her 'Revival' tour still to hit Europe and South America.
"There's real concern and the plan is to talk to her in the next few days to ask her to consider taking some time off."
During the spat, both Selena and Justin, 22, each accused the other of cheating during their on/off romance before Selena took to Snapchat to say sorry and sources close to the star claimed that she only hit out because she is still hung up on Justin.
A source told PEOPLE: "He was Selena's first love, and she hasn't been able to totally get over him."
During the online argument, Justin also accused Selena of using him for fame, but the insider said: "They're both young and have made mistakes, but she has never used him. He put their relationship out there way more than she ever did, and he's been very strategic about it. Whenever something good is happening in her life, he'll post something to get at her."