Sansa shrugged her off when Arya first told her about her list, thinking she was just an ignorant child. When she saw Arya is a better fighter than her best soldier it made it real for her, like oh shyt Arya is not to be fukked with. Littlefinger was probably still thinking of her like a child until he saw that, he may recognize that training.
Bran and Arya both leveled up while they were away. She might be feeling a type of way.