"Teen Mom" stars Catelynn Lowell and fiancé Tyler Baltierra might not be getting married as soon as you think ... TMZ has learned the duo has joined the new season of  "Couples Therapy."

Catelynn and Tyler -- who infamously put their "Teen Mom" kid up for adoption back in 2008 -- have been on and off for over seven years -- but were reportedly set to tie the knot in Michigan in a few months.

Now, sources tell us, Catelynn and Tyler will hash out their problems on the Vh1 show ... about couples whose relationships are floundering.

We're told rapper Chingy, Flavor Flav and Joe Francis have also joined the cast ... with their current significant others.

Sounds like a recipe for ... reality TV.

"Teen Mom" star Catelynn Lowell -- the girl who gave her baby up for adoption on the show -- got into some serious HIGH jinks this summer, ripping a massive bong at a party with friends.

The photo -- obtained by TMZ -- tells the story.  It was snapped last month at a friend's get-together in Michigan -- showing Catelynn in rolled up jeans, igniting some sizable glassware ... and taking a big ol' hit.

Catelynn could've been the one "Teen Mom" star who ever appeared responsible and smart.

Past tense.

Catelynn's manager, who said he could not "confirm the validity" of the photo, tells TMZ, "Like many other young adults, Catelynn, at times, struggles with making the best decisions for her health and career. As her manager, I have strongly advised her to exercise good judgment at all times. She must learn from her mistakes. This will serve as a teachable moment in her development as a young woman."