The back-and-forth bickering from last week's premiere of "The Kardashians" bled into the weekend -- with Kourtney taking to Instagram to respond to fans who wondered if Kim's on-screen suggestion of a "Not Kourtney" text group was actually true or not.

According to Kourt, it's BS ... at least based on some digging she's been doing on her end. KK responded to one person, writing ... "I believe it was just my sisters from the surveys I've been taking."

She added this while replying to yet another fan who inquired about it ... "the members of that chat are namely Kim, Khloe, Kendall and Kylie. Case closed." It looks like Kourtney checked in with her actual friends ... and started a new group chat with them.

She posted a screen shot of their conversation ... and it's pretty clear they're pro-Kourtney. Also, folks have apparently been contacting these pals of hers and asking if they're in the alleged "Not Kourtney" group -- and they all seem to be denying it.

Of course, the reason this is even being discussed is because Kim alleged on the show that some of Kourtney's buddies have been secretly cozying up to her and other family members to air their grievances about Kourt.

Kim made it seem pretty dramatic, too ... saying, "- "All of your friends call us complaining. Whether you think they're the ones going to you, they're all coming to us on the side, saying the opposite to us."

She added, "We're all confused, and we're on a group chat labeled 'Not Kourtney' so we know, and have to funnel what your friends are saying to us, and have to figure out why you're such a different person and why you have this vendetta."

This was part of a much larger argument the two siblings has over the phone ... in which Kourtney accused Kim of being a narcissist and always wanting to steal the spotlight.