it definitely wouldn't hurt to call your doctor. Do you feel this same numbness when you walk outside or just on the treadmill ? One thing to consider is whether or not your shoes are tied too tightly. Numbness or any tingling could be a sign of lack of blood circulation to the feet. So, if you're wearing old shoes or the laces are too tight, that might be the problem.
One thing I suggest to people starting a walking or running program is to make sure they are wearing properly fitted shoes. Ill fitting shoes can cause a bunch of problems.
Here's a SP article on fitting shoes that you might find helpful. www.sparkpeople.com/resource/fitness_artic