Either one of two things:
1. Like Evo stated, that she didn't mean anything by it and was tying to be friendly.
2. She's intentionally trying to get you frustrated. This is fun for some females. Show her that you have the hots for her, and you are handing over power to make you feel good and bad... This is probably not the case, but it's possible she's delivering some old-fashioned torture. Females are very clever, when they are young it can come across as cruel. I know that I feel bad sometimes reflecting on the things I did to boys. I considered it retribution at the time though.
I think you should let it go and focus on women, not girls, when you become a man. What is the point of lusting when you are young and it's highly unlikely that something will come of it? Be nice to girls, but don't take it personally when they act that way, and start focusing on yourself. People are people, no sense in worshiping them to the point that you feel bad about yourself!