Jenna Ortega's promotion for her new flick's a real pain in the behind ... literally, in fact, because a dress she wore for an interview included glass poking her booty!

The actress stopped by "The Late Show With Stephen Colbert" Tuesday night to discuss her new movie "Death of a Unicorn," as well as Season 2 of the hit show "Wednesday" ... and, she showed up in a fiery red gown covered in shiny material.

Turns out red glass gave the dress its shine ... which ain't the most comfortable material to turn into haute couture.

Check out the clip ... JO says sometimes there are pieces of the dress stabbing into her -- and, Stephen jumps in and says they should have medics standing by.

Ortega adds she's uncomfortable in the dress ... but, strangely comfortable at the same time.

It's a big few months for Jenna coming up ... the star-studded flick "Death of a Unicorn" -- costarring Paul Rudd and Will Poulter -- is set to come out this week, and Season 2 of "Wednesday" will come out later this year. So, expect to see Ortega wearing more of these uncomfortably comfortable get-ups in the coming months.

Wednesday Addams may stick to basic black ... but, we gotta say calling a dress forcing glass into her behind "comfortable" is a total Wednesday move.