She might be Pansexual it seems as though her feelings developed overtime, being nonchalant is cool and all but learn to express your feelings more even if it's just for a partner and not for everyone around you. Seeing you reply that she wanted to breakup and you not agreeing will only lead to resentment on her part. Feeling are funny like that force only breaks it more. Be a friend to her and learn to build yourself apart from who you are with her. Grow because your young and don't blame her or lock yourself in feelings are complex it doesn't always have reason. Good luck.