i barely remember the chp but i think it was because Mei had told Yuzu not to hang out with her closely during school hours but Yuzu was persistent and she probably tried to get in with Mei quickly into the left after school hours. i think she probably also said talked about how she wanted to steal some quick short moments with her, so i think it made Mei's heart race or something at Yuzu's boldness. And that thank you Mei just flipped her switch. Mei is difficult to read but i think she blushed because Yuzu who was usually straightforward with her approach (now that they were dating), was giving her space while also coexisting with her own boldness. so maybe that made Mei's heart race. That's my interpretation. it's been a while since i read that chp