Carrie Underwood -- SELF-ASSBUSTIFICATION ... Falls On Stage, Hurts Foot

Carrie Underwood learned a painful lesson on stage in Corpus Christi, TX last night ... 5-inch stilettos + a long, flowing shirt = RECIPE FOR PAIN!!!

The country star was performing her song, "Undo It" ... when she kicked her heel back and it got CAUGHT in the tail of her shirt and -- BOOM -- she was on her ass.

Being the pro that she is, Carrie continued singing like nothing was wrong ... barely missing a note.

Later, she tweeted: "Hey, remember that time in Corpus Christi when that girl busted her butt on stage?! Hilarious! Oh, wait, that was me!"

But it wasn't her butt that was busted ... it's her foot.

Good news, though -- it's not broken ... just sprained and last night, Carrie posted a photo of her new footwear.

Get well soon!