While You Were Sleeping (2008)

Yu Jin is a single girl with lots of boyfriends. One morning, she wakes up and finds herself in a hotel room with a faint memory of having slept with a man in the previous night. She had a wild party and got drunk to the point where she couldn't tell whom she was with, so she makes a list of several potential suspects. Cheol Jin, who has been one of her best friends since college, is among them, but she's convinced that it couldn't be him because he's too shy for anything like that.