It's true -- Brittany Matthews is a budding social media fitness star -- and she showed off the fruits of her labor during an awesome girls trip to Hawaii.

Brit -- along with a bunch of her girlfriends -- flew to Hawaii for a tropical bachelorette party (sans the Chiefs QB) ... where they partied and hit the beach in tiny bikinis.

In fact, all of the girls on the trip had custom swimsuits with their nicknames on them -- Brittany's read, "Fit Brit."

Don't worry, besides relaxing in paradise, Brittany found plenty of time to exercise -- hiking and hitting the gym, because NO DAYS OFF!!!

By the way, Mahomes and Brittany have been together for years. They started dating back in high school and continued dating while she was attending the University of Texas at Tyler and Patrick was at Texas Tech.

In fact, the two were just spotted all over L.A. a few weeks ago, hitting Lakers games and all the finest Hollywood hot spots.

It all begs the question ... who gets a ring first -- Pat or Brit?!?!?