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.