Her comments -- which ranged from implying he cheated when they were together to suggesting he had ulterior motives in posting cozy photos of himself alongside rumored new girlfriend Sofia Richie -- were sparked after Justin's fans began complaining about seeing him with Sofia. She's since apologized, posting a Snapchat that simply read, "what I said was selfish and pointless."
But why did she get involved in his Insta-issues to begin with, two years after they ended their high-profile relationship for good?
Insiders tell the New York Post neither member of the couple formerly known as Jelena can "Let It Go," Queen Elsa-style, because they still have strong feelings for one another, even if they know better than to act them romantically.
"He was her first love, and he still has a major hold on her. If he texts her, her world stops," says one source.
Another agrees, adding that Justin "is not over" Selena either. "They came up in this business together as sweethearts — those feelings will never go away," the source says.
Instead, those feelings appear to be turning negative. Selena first chimed in on Justin's pics of him and Sofia over the weekend after her threatened to make his Instagram private and tossed out this playground-worthy barb: "... if you guys are really fans you wouldn't be so mean to people that I like."
Enter Selena, who commented: "If you can't handle the hate, then stop posting pictures of your girlfriend lol- It should be special between you two only," she wrote. "Don't be mad at your fans. They love you and supported you before anyone."
Cue Justin (and maybe an eye roll, we won't hold it against you): "It's funny to see people that used me for attention and still try to point the finger this way. Sad. All love," he wrote, before adding, "I'm not one for anyone receiving hate. Hope u can all be kind to my friends and eachother [sic]. And yes I love my beliebers."
Like a ping-pong ball at the Olympics, Selena snapped back: "Funny how the ones that cheated multiple times, are pointing the finger at the ones that were forgiving and supportive, no wonder fans are mad. Sad. All love."
Things went downhill from there, with Justin invoking a Zayn Malik reference that may or may not have been posted by him (since everything has since been deleted, we're relying on screenshots from the super reliable Interwebs here; TMZ says Justin did not post anything about Zayn).
Following her semi-mea culpa on Snapchat, TMZ reported Selena tried to contact Justin repeatedly but he refused to respond, allegedly because "he feels like his ex won't let go, and she's constantly looking for ways to insert herself in his biz," according to the gossip site.
TMZ adds that Justin "is very over" Selena (cough) and thinks her initial criticism was hypocritical since she threatened to quit Instagram in February.