She invited Dr. Jacob Unger -- a plastic surgeon in Nashville -- to chat for an episode of her "Let's Be Honest" podcast, and during the convo, she admitted having a breast lift and breast implants.

Kristin let the info slip after asking the doc if he ever has to turn people's requests down ... and, he revealed it's a common problem with some wanting a similar treatment to a friend without regard for differing body types.

For instance, Unger says people will ask for the same size implants as a friend, even though their bodies aren't the same size. That prompted KC to say she's not sure how many CCs she's got in her chest, but the number works for her body type.

In fact, watch the vid for yourself ... Kristin's saying the average listener probably needs to ask for more bust than the size she got herself!

And, in case anyone thought she mixed up implant surgery with a breast lift -- a procedure she previously copped to having -- Kristin doubled down in her Instagram comments by firing at a fan who seemed to question why she hadn't revealed the info before.

She says she got the implants after breastfeeding, and added no one ever asked her point blank if she had breast implants, and she never lied about it. So, sounds like she's not taking plastic surgery shaming.

Kristin also opens up about BBLs in the podcast -- and, you've gotta see what she thinks about possibly getting the procedure herself.