Why she said wow after I kissed her?

We have been friends for a year and we hangout sometimes and occasionally talk on the phone.

She called me at night and asked me to pick her up and take her home. At the parking lot in front of her building we danced close to each other. while the music was playing from the car speakers. She asked me inside of her home to sleep but then was unsure and said maybe another time, but then leaned on me to kiss my cheeks and I said her kiss me again on the cheek and immediately after that I kissed her on the lips and she said wow. I apologized and she just smiled until I drove away.
Why she said wow after I kissed her?
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