I'm gonna make a guess you're in school?
In that context, probably she wanted to get a reaction out of you. It could be that she is a bully, but if she is someone you get along with, or a friend, she may think you're interesting and want to see your reaction.
It sounds silly, but sometimes people pinch or poke people they like, just you see them react.
She probably wasn't expecting you to spank her back, but she was the first to spank, so your retaliation was understandable.
If it continues and she keeps spanking you, and you don't want her too, you should ask her to stop, and if she keeps it up, report her.