I've noticed exactly the same experience in recent years with common colds (which I've had slightly more often than normal thanks to young children). Like the parent post, the day before I feel "unusually well—almost high", and the next day the cold symptoms start, or I get a day of feeling especially unwell without any specific symptoms (the blearghs). Don't think I've read anything supporting this, but in my layperson's mind it is due to my immune system going into overdrive trying to fight the infection.