Why it hurts more when you're cold.

Imagine your body is water. If the water is warm, you can't break it and if you drop a penny in it, it absorbs it. It will still cause impact but the energy will be spread out and it causes ripples. Now if you freeze water it gets hard, it doesn't absorb the penny. The penny hits it and causes a lot of impact because the energy doesn't spread.

So when you're cold, you're skin is more hard and the energy from a punch is more localized at the point where you're hit. More energy means more pain.