The source explains, "Brian is annoyed that she is so focused on her relationship with Machine Gun Kelly."
"He doesn't understand the need to post social media pictures with silly captions to proclaim her love for him. They aren't even divorced yet," the insider continues.
That't not the only reason he's upset. The source says, "And it stings a bit too that Megan moved on so quickly. Megan was always very private, so Brian just doesn't get what changed."
He definitely made his frustration on social media clear with a passive aggressive post that got everyone talking.
The social media tension started when Fox posted a selfie of her and her new beau. She captioned the post, "Achingly Beautiful Boy... My heart is yours."
This is not the first time Fox has been open about how much she cares for MGK. During an interview on the Give Them Lala ... with Randall podcast, the actress called him her "twin flame," explaining that they are "actually two halves of the same soul."
Hearing your wife call someone else her twin flame has got to hurt, especially when you were only recently separated.
So Brian called out the new couple on his own Instagram, posting a series of pictures of his sons, and copying Fox's caption. He wrote, "Achingly beautiful boys...... My heart is yours."
Yep, there's definitely some tension there.