The proverbial "little voice" in every girl's head that makes her question everything, decide nothing, react irrationally (but justifies the reaction with wicked jacked up logic), over think, and causes 98% of all drama in her life. The girl brain is all of the bad things that have ever been said to you and that you believe about yourself, it's your baggage surfacing and dictating how you interact with others.
He said - "I'm sorry, I am really busy tonight, can we do it tomorrow?"
Her girl brain said - "What is HE DOING? WHO IS HE WITH? What did I do? Is something wrong? Something must be wrong, is he seeing someone else? Probably that red-head from the coffee shop. I think I will go eat a gallon of icecream and watch lifetime."
What he meant was "I'm sorry, I am really busy tonight, can we do it tomorrow?"