What the heck is up with my daughter?

The last few nights she has not wanted to go to bed. She has been up until 10pm or later. Her bedtime is 6:30pm and I've put her down at that time each night. She just doesn't stay in bed. I have to put her back to bed a dozen or more times each night until she finally falls asleep. Why has she turned into a night owl? This is not a good thing. Mommy isn't a night owl!! I really hope that this changes really really soon.