There are so many factors involved.
Factory ammo or reloads that's using temperature sensitive powder. The temperature when you sighted in and/or worked up a load. The difference in the atmosphere and heat / cold currents of air.
If you reload then using temperature insensitive powders like ADI's should even out the playing field but 45c is getting a tad warm.
I've often shot / tested early hours of the morning when it's about a few degrees then warm afternoon around 30c and notices very little difference.
The biggest affect I've found is shooting targets at 500 metres across deep gullies and noticed the POI change quite a lot due to the air currents moving down the gully from hot to cooler ground. So your accuracy change might have numerous reasons for happening depending on distance as well.