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.