[ukmicky (OP)]

When your skin gets cold it feels numb ,so why when you knock your self does it hurt more than when your warm.

[paul.fr]

possibly, because when part of your body is numb you have a restricted flow of blood to that area. as such your nerves around that area are more acute.
just a guess.

[ukmicky (OP)]

Could be right but i would have thought that anywhere with a reduced bood supply would also feel numb like when you sit on your leg funny or lean against your arm and your hand goes numb.