ICYMI ... fans of the first ever 'Bachelorette' were sent into a tizzy when Trista's husband, Ryan Sutter, posted a cryptic message on social media about missing her on Mother's Day ... playing it very coy about her whereabouts.

Trista eventually resurfaced online, reuniting with her family in Mexico and giving an equally vague message about her mysterious disappearance. The whole thing was bizarre -- but now TMZ has learned exactly what was behind it.

Sources with direct knowledge tell us ... the reality TV legend wasn't "missing" after all -- instead, we're told she was filming season 3 of Fox's "Special Forces: World's Toughest Test."

For those unfamiliar with the show, 'Special Forces' takes a group of celebrities and has them undergo grueling challenges from the playbook of the U.S. military's selection process.

The show's become an opportunity for celebs to take a hard look at themselves while putting their physical and mental fortitude to the test... with "Vanderpump Rules" star Tom Sandoval, 'Bachelor' vet Nick Viall and "Dance Moms" alum JoJo Siwa all giving it a go in season 2.

This explains why Trista said she'd gone through a lot of personal growth and a journey when she resurfaced -- and Ryan played into that as well with his social media post, which was, obviously, carefully crafted to conceal the casting news.

BTW ... 'Special Forces' is going deep with Bachelor Nation, because our sources also say Ali Manno, formerly Fedotowsky, will also be on the upcoming season.

Fans of that former 'Bachelorette,' from Season 6 in 2010, might have noticed her posting lots of intense workouts in recent months -- including boxing, weight lifting and cold plunges. So, now we know why she was getting so physical.