Why Am I Obsessed with a Friend?

I recently transferred to a new school where I knew close to no one. I made a few friends gradually, as a result of joining the school volleyball team. one very kind girl went out of her way to include my in activities. We started to hang out, and we sometimes hang out on weekends, but always doing an activity. Whenever we are simply talking to one another, I freeze up and lose my personality. Then, when we aren't together, I can't help but obsess about every interaction we have. How do I act normally and calm down? Also, is this obsession normal? Thanks in advance !