If you decide to fuck her, game sets "hsww" variable to True. Then, when she messages you mid-game, that variable is used to show the messages in which she talks about that sexual meeting (if you didn't fuck her, she just tells you she's still at the dinner and didn't went to college). And the next time that variable is used is to trigger all her messages in last chapter, when Mabel leaves you alone. If you didn't fuck her, you don't get any message from her even if you replied to her the other day.
So no, either you did it (maybe just for the sake of watching the scene) and forgot to rollback and keep playing from that point instead, or you're using some kind of mod or something that's messing with that variable