What shall I say to my girlfriend?

My girlfriend just annoyed me. Here's the Convo:

GF: by the way, I have to confess something. Hope you will be able to forgive me.
ME: What?
GF: I did something horrible
ME: Such as?
GF: I watched (romantic movie) without you
ME: I thought it was something big
GF: I was just mocking you
GF: Did you get upset?
ME: Not really
GF: I mean what else could it have been?
ME: You know what
GF: No, I actually don't. What were you thinking of?

I thought she was going to confess that she kissed another guy or something. The way she built it up was as if she did something really bad! How can I answer her last question? Shall I tell what I thought she was confessing? I really hate those kind of games GAHH!
What shall I say to my girlfriend?
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