Let me guess

Sori needs the girls help to do some kind of surprise to Iseul, not noticing that she was indeed pissed when she left. Iseul falls asleep while drinking with her ex, do not fuck her but wake up in her bed, the woman messes with her and said they did it. Iseul feels guilty and hangs on the fact that Sori didn't choose her to act normal, but when she realizes why Sori went with the girl she cracks. They fight then end up dating.