I read a fic years ago that was about Kurt getting drunk at a party and accidentally sleeping with her. She ends up getting pregnant from it and from what I remember, she still tells Finn that the baby is his before Kurt finds/figures out. Quinn ends up getting kicked out of her house and she moves in with the Hummels while she's pregnant. I remember Kurt wants to get married to her just because of the baby, and I do remember Quinn smacking him at one point.
I would love if someone could find a link to that story. I'm almost positive it was in FFN, but I can't be sure. Thank you for your help!