Yep! Had you been busier/more active than usual in the days before the crash? With ME, our bodies can't handle certain thresholds of mental, emotional, or physical activity.

Agree with first commenter. Did you do anything out of the ordinary? If I go on a big hike, or do Spring cleaning all weekend, boom -- I'm exhausted for a week! Or it can come after an especially stressful week at work. It's hard because sometimes you feel really good and you want to make the most of it, but you know you'll pay. But I hate wasting the good days.