Kristin Cavallari Goes Public with New Model Boyfriend After Cabo Vacation

3:08 PM PT -- Kristin Cavallari just took her relationship to the next level, going IG official with her new boo in a cute selfie. She captioned the post, "He makes me happy."

Kristin Cavallari has a new stud in her orbit -- and it looks like she brought the young buck with her on vacation south of the border ... and TMZ has the first picture of this hunky fella!

Check out this photo we obtained, which was taken last Friday down in Cabo -- where we're told she'd just landed with dude ... whom we've identified as 24-year-old model Mark Estes. Eyewitnesses tell us this is them on the tram at the airport in Cabo heading to customs.

Even before this, though, we're told Kristin and Mark were very flirty on the flight over -- showing a lot of PDA ... and certainly looking like a couple.

Sources familiar with the situation tell us Kristin and Mark have been spending time together for about a month now -- but it's unclear if things are exactly romantic or not.

We're told Kristin was down in Mexico last week to work on her Uncommon James jewelry line ... and our sources say Mark tagged along for the quick trip. No word on how exactly they met ... but we're told they're hanging out right now, and that it ain't that serious.

Remember ... the last high-profile relationship Kristin was in -- post-Jay Cutler, of course -- was actually Tyler Cameron ... another model who's hobnobbed in and around Hollywood.

Doesn't look like Mark's as famous as him ... but he's definitely just as good-looking.