I'm such a stupid person. Tell me what my mistake was?

We were walking out of cl and this girl asked me if I had a cl. I told her at twelve. She said she had a cl also at 12(it was 11:15am when she asked). She also said that she was going to buy something from the caf and then go and eat outside since it was pretty warm outside. I said that was a pretty good idea and said good-bye. I only realized the mistake I made in that case. Tell me what you think is my mistake cause I want to be sure I’m right.