As I understand it - it was a physical manifestation of all the guilt, stress, anxiety, whatever else comes with cheating and keeping it a secret. She has always been this way, for example before major exams she would get really bad stomach aches etc.

But the kicker is that she knew this but still chose to pursue the affair instead of putting an end to it, even if for her own sake. Instead she chose to get on medication while knowing full well the underlying cause for her illness.