If I'm reading it right, you haven't ridden fully loaded for a while? Even if you commute, ride unloaded etc, it may take a day or two to get back used to the load. You may have just pushed it a bit more than you're body was used to - in combination with the heat, and then using up energy reserves, it doesn't sound surprising at all to me.
Fully loaded climbs are relatively much slower in my experience. Takes a paradigm shift to realise "oh, I have to go this slowly to conserve strength"
Maybe you just tried to go too fast (at your normal speed) up those hills.
... then again, it could have been just one of those days
Whatever, don't worry, sounds like all you need is a couple of days back on the road and you'll be back in form...
Much of this.
But wind also can take a lot out of you without you noticing. Especially in warm weather because the evaporative effect from the wind seems to keep you cool and you don't really notice how much sweat you've lost.