What bad memory haunts Kristen about the trip she and Kristen took to Cambodia in the novel, We Were Never Here?

Emily and Kristen take a trip to Cambodia about a year before the novel begins. They have a lovely time exploring the beautiful country. Emily feels so relaxed and happy that she even takes a man back to her hotel room. She and Sebastian begin to make out, but then he sexually assaults her. She fights him off and then Kristen barges into the room and helps Emily. They kill Sebastian and take his body up to a cliff where they dump him over the side and into the ocean. Emily hates thinking about Cambodia and is terrified that the authorities will find out what she did.