Having lap times would be a great help. As stated, cooler temps would mean more dense air and more bit from the prop, and then faster lap times. It just may have seemed a lot faster to you if you have never seen this change before. I suggest that you not change anything until another sunny day comes along, and see if the resulting flight is like "normal". If it's significantly warmer, be ready for the lap times to be slower. If it's cooler again and the lap times are still quicker, try a slightly smaller prop or one with slightly less pitch and see how it performs.
Type at you later,
Dan McEntee