Post Traumatic Stress.
Chell was past that point in her life. It was years later; she had begun to forget about the Aperture Science Lab. The scars and burns she had obtained had begun to fade, and she no longer jumped whenever some one said 'who's there', or 'there you are'. She was able to eat cake again, not just sit there picking at it unappetizingly.
And every time she thought she saw red beam around the corner, she reminded herself that she had destroyed the lab.
But mostly, she no longer thought she heard GLaDOS everywhere.
She had gotten past all that. It was over. She had won.
It was one evening, when she was at home, relaxing and sipping a glass of wine with one of her friends, (she had made friends, when she had gotten back, just to prove GLaDOS wrong, because if she was right then that would be a victory, and Chell had to beat her completely, in order to ever get a good night's sleep again) when she heard a ding that accompanied a new e-mail.
She went up to go check it, and her heart nearly stopped when she saw the send date.
Three years ago, the day after she had escaped from Aperture Science.
Her pulse was racing fast as she opened the message, and reads the first lines.
"To: enter subject name here
This was a triumph…"
Chell drops the glass of wine and screams.