Sources close to the situation tell TMZ ... Kristin broke up with Mark this week because their relationship had run its course and was never intended to be forever.

On Friday, Kristin briefly mentioned the split to podcaster Alex Cooper while they chatted onstage during one of Cooper's "Unwell Tour" stops in Austin, Texas.

You may recall ... Kristin and Mark first got together in February 2024 — with Kristin making them Instagram official.

The reality star/fashion designer posted a selfie with Mark and wrote in the caption that he made her happy.

From that point forward, the two were hot and heavy and seemed like a serious couple, but our sources said they never reached the stage of moving in together full-time.

In early September, Kristin said while she was having great sex with Mark, the two were not talking about having kids or getting married.

But, Mark -- a co-host of the popular TikTok show the "Montana Boyz" -- had previously told Kristin on her "Let's Be Honest" podcast that his goal was to tie the knot someday and have children.