stimulants cause vasoconstriction but meph is particularly good at it for some reason. the more you take the worse it gets. I would bet you are redosing over and over to stay high and intern cause severe vasoconstriction...that is what is making your hands numb.
is it normal....kinda...it is certainly not healthy. the vasoconstriction fom meph has been the cause of numorous hospital visits and several deaths.