I get pins and needles a lot as I have Raynauds disease.
You talk of a "constant" numb hand.
Does your hand change colour during these pins/needles attacks? I mean, like, does it go very pale, to blue/purple, then a burning red colour? If so, go to your GP and explain the symptoms. If they're concerned they'll probably run some tests for Raynauds disease.
Btw Raynauds isn't scary or life threatening so don't worry, and I'm not intending to worry you! Chances are you won't have it, it's very rare - but mild symptoms are common in the majority of people.