ORIGINAL: Lownverted
I think Crazy has the right idea. Sounds like the change in heat and humidity either caused the airframe or the control system or both to move and make it go out of trim. Not an uncommon thing with wood planes.
The aircraft flies, stalls & does everything else at the same airspeed no matter the density altitude. What changes is how those speeds look to you.
Try explaining that to a 747 pilot taking off at Sky Harbor in Phoenix in July at 116 degrees.