Kourtney Kardashian says she's at that phase after havin' a kid where nothing seems to fit ... so she's showing off her holiday postpartum outfit, and get gettin' tons of praise for it.

KK opened up about her dilemma Tuesday, the day after Christmas ... showing off her holiday fit, which consisted of a bodysuit, tights, a fur coat, and a whole lotta bling -- and according to her, the coat wasn't picked out randomly.

The caption reads, "When not much in the closet fits yet and the boobs are filled with milk, throw on a cozy coat." Folks in the comments have been hyping her up, too -- including her sis, Khloe, who wrote, "When you’re a baddie no matter what."

As we reported, Kourtney had her baby, Rocky, last month ... and the bump was shown off quite a bit leading up to the big day, including in a racy photo shoot done for Travis' bday.

She also had her pregnancy belly on full display at Travis' Blink-182 concert, which let the whole world know they were expecting.

You'll recall, Kourt and Travis showed off a first look at baby Rocky on Friday -- hiding the kiddo's face, but showing the happy family cuddling up with the little one.