The 28-year-old shared an image to Instagram over the weekend of Rosamund Pike's character in the film Gone Girl alongside the caption: This is my sister kim, the bitch from Gone Girl,,,"
There was no further info and the post is still visible on Rob's Instagram page – the only post on the entire page.
Rob's random picture has sparked concern among fans, but a source told People that Rob didn't mean anything with the statement.
The insider said: "He's so bizarre, he posts and deletes and thinks people aren't going to notice."
Rob – who has nearly 3 million followers on Instagram – is known for his love of posting and deleting. His whole Instagram page only has one picture currently – the Gone Girl one.
Meanwhile, Kim has not said anything, instead focusing on tweeting about the latest episode of Keeping Up With The Kardashians, which aired last night.
Earlier this month, the mother-of-one shared a heartfelt and very public birthday message for Rob via Instagram.
Despite being a member of the high profile Kardashian Jenner clan, Rob has remained away from the spotlight for the past year. After ducking out of Kim and Kanye's lavish wedding in May 2014, he's stayed out of the public eye.
Over the weekend, sister Khloe, 30, suggested that her only brother is struggling with social anxiety.
She told Wonderwall: "I just feel, especially over the last year, Rob has become very introverted and has a kind of social anxiety. He's definitely not at his happiest place that he once was, and I know he can get to that happy place and he will."
All you need to know about Kim Kardashian!