I bought tickets a month or two ago. I was on Telecharge looking for the best availability but eventually went to the box office to buy the tickets. Didn't see anything about scheduled absences on the website & they definitely didn't mention anything at the box office. So I assume that was just an unplanned absence.
EDIT: Yeah. She tweeted yesterday at 5:30pm about getting influenza. Oh well. Wish I saw that yesterday but not sure if I would have been able to exchange. Hope she feels better though.