I just tell them that's the way I am, but that I'm having a good time.
They usually stop bothering me or change the subject.
I don't have to explain to everyone why I'm more reserved or introvert, no matter how curious the person is,
unless it's someone I can really trust.
I still get this from time to time, like many I tend to think of something snappy moments later. My solution would be to just have a phrase on hand since you know people are going to ask (i.e. "I don't know I'll have to think about that")