What Causes Pins And Needles In Hands And Feet?

I have pins/needles feeling in my hands and feet. but not all at the same time and you never know which place it will happen. sometimes just one foot. sometimes just a few fingers in one hand. sometimes just top of foot and whole hand at the same time. no rhyme or reason to it...comes and goes. usually lasts at a minimum of 24-48 hours and a maximum of 5-6 weeks. any ideas?