[Ofir]

Humidity adds weight to the reeds, as it condensed on them. More weight results in slower vibrations, hence the lower frequency you got. The reeds are VERY light, so even small drops of water may cause significant changes.

I quite doubt it that the actual temperature of the reeds has such a dramatic effect, but you can perform a simple test:
Play on it for the amount of time you mentioned, and then leave it lying for ~5 minutes without applying any humidity extraction process. The reeds will for sure return to room temperature. Next, you can test again to see if it’s the temperature change (that was now dismissed) or the humidity (which probably stayed).