@delaprd she was always just as embarrassed by her emotions. that is why she teases and always goes on the attack. A good offense is a strong defense. Remember how embarrassed and happy she was when she found the notebooks? Or when she grabbed his crotch? This is her when "her armies and offense is smashed and she has no defense to fall back on." if you will.
edit: also, you got to look at the MC from the whole series. In the beginning, he couldn't even look at the girls' faces. This one he went down and successfully talked with a girl he didn't know to find out her class. He even worked up the courage to attempt to say her first name. His reaction is pretty reasonable given those 2 friends always tried to really mess with him when Nagatoro isn't around. And once you start to panic, your decision making skills go right out the window, so i can see this all happening. He even can tell he is being stupid, but sadly the dice is cast at that point.