In a later episode, she realizes that giving her umbrella to a man sitting out on the street, that she had expected thanks in return. Sensei congratulates her for learning at such an early age that she should do good things with NO expectation of even thanks.
So, later in the same episode, she helped two of her classmates. When they thanked her, she told them that she wasn't doing it for them and ran off. They misinterpreted this as being Tsundere.
I like her either way. Worried or Tsundere.