There's more to that Jay Cutler butt pic than meets the (brown) eye ... so says Kristin Cavallari

Turns out, the famous Jay Butt-ler pic was taken during an outing to a nude beach during a romantic trip to Mexico back in March.

Cavallari told the story to our friends at TooFab ... saying they got caught up in the moment and figured, "When in Rome ..."

KC says she still can't believe the Miami Dolphins QB let her post the photo on social media!

TooFab also asked how the family felt about Jay's new 1-year contract with the Fins and if he plans on playing next year!

Make sure to check out the rest of Kristin's interview about her jewelry line, Uncommon James, on TooFab!