There is a HUGE difference between "being used to" heat and humidity as a resident where 95% of the time you're in air conditioned comfort (malls, mrt, bus, home, office), compared to "tourist life" where you're walking up and down the entire day visiting temples and stuff.

My first japan trip was to Tokyo in summer and by the end of the trip my luggage was pretty damn stinky cos of all the unwashed sweat on my t shirts.