I think Kimura really did have a crush on Mizushima; she did get nervous whenever the two of them were alone together, not to mention this page. That said, when I reread the manga recently, I noticed that she's also always looked at Hiyama in a positive light. Even at the beginning, she admired her and wanted to be friends. For example, when her friend said she was Hiyama's rival, Kimura's response was "That's way too presumptuous!"; when Mizushima said he was surprised they were friends, Kimura thought "Does that mean it's strange for someone perfect like Hiyama-san to hang out with a plain person like me?"
To sum up, it's definitely possible Kimura was crushing on both Hiyama and Mizushima from the beginning, and was just focusing on Mizushima due to some combination of comphet and Hiyama's various antics scaring her. Now that she's gotten to know Hiyama better (and also gotten used to her...*unique* personality lol) she's starting to realize it more. It's been building since the ice cream "date," too; see here and here.