One of the chicks Tristan Thompson made out with back in October says she had no idea he was serious with Khloe Kardashian ... much less that she was having his baby.
The woman's name is Marie -- she's the one on the left in the surveillance video -- and sources close to her tell TMZ ... Marie was given the impression Tristan and Khloe's relationship was casual. We're told she later learned Tristan and Khloe had been together for almost a year ... and, at that point, she cut off the fling.
Our source says Marie is a mother herself, and did not want to be a homewrecker.
TMZ broke the story ... Tristan cheated with Marie and another woman at a D.C. area hookah lounge. The video shows them making out and getting very handsy ... with Tristan's junk.
For what it's worth, that encounter went down 11 days after we broke the story Khloe and Tristan were expecting their first kid.
And here's a crazy un-related fact -- we're told Marie was at the Miami party last month where Chris Brown grabbed a woman's neck. She's the one in the floral dress. You'll recall, Chris said it was all just horseplay, and we're told Marie says the same.