Why is it that every time my feet or legs "fall asleep", it feels like I'm walking on needles?

Confused nerves. The activity that caused your feet to fall asleep actually cut off or severely impeded nerve impulses from your foot to your brain as well as cut the blood flow to the involved nerves. When you began walking the nerves need time to readjust to normality and until that is done, the nerve signals will be scrambled. This nerve "confusion" causes the feeling you are having.