Khloe Kardashian's 'Dad + Mom' Pic with Tristan Thompson was NOT Pregnancy Announcement

Khloe Kardashian's birthday pics with Tristan Thompson -- which she captioned "Dad + Mom" -- were NOT some sort of secret pregnancy announcement ... TMZ has learned.

Our Kardashian sources tell us ... her social media post on June 25 is purely a coincidence -- she had no idea she was pregnant at the time. We're told Khloe's just shy of 3 months pregnant now ... and the surprise 33rd bday party Tristan threw her was 3 months ago.

We're also told "Dad" and "Mom" are just lovey dovey nicknames the couple have for each other ... though they now have a deeper meaning.

Want more proof Khloe didn't know she and her bf were going to have a baby? Here's what she tweeted the day after the party ...