All sorts of things can kick up your blood sugar. Stress, excitement, sickness (sometimes you get a virus that your body fights off without you getting any symptoms), temperature...
Don't worry if you get an inexplicable high, or even a bad day or two. Try your best to find a cause so you can learn for the future, but don't let it get you down. If you are getting highs consistently, THEN it's something you have to address.
Cheers