Could be health related.
Nope. NationalJournal.com is reporting that she is set to become the President of the Woodrow Wilson Center on Tuesday. To me, that's pretty crappy - run for reelection, win and then less than a month after the new Congress starts, you quit. Now, at considerable cost to the taxpayer, they'll have to be a special election.
Her campaign should have to foot the bill for the special election.