Ah, I see what this is. Each episode they'll rotate who gets close to Serinuma. At first, we thought this might be Igarashi's episode but they quickly showed us Mutsumi was the chosen one.
Maybe a little too much to have him undress her. The almighty and dangerous Japanese-rain-induced-fever. Can present symptoms a few minutes after getting drenched and will come to pass with a warm blanket, and a hug