you don't say how long have you been running?  Did you get your shoes from a running store?  I hear that's a good idea.  LOL  I'm assuming that you did.  I wear barefoot running shoes and I love them, if you haven't tried those you might them them a shot.  I've gotten numbness in my feet all 3 times I started C25K.  I also had awful leg cramps and I'm sure the numbness was caused by those.  So my question would be could you be getting a cramp in your foot that is impinging on  a nerve?   The 1st time around I was  wearing cross training shoes and sure it was the shoes because as soon as I switched to the barefoot ones in week 4 the cramps started easing.  Only when I had to restart after a back injury I had the same problem with leg cramps in the 1st 4 weeks.  Knee injury, restart and awful, painful cramps again, which started easing in week 4.  I've tried all kinds of stretches and got tips on how to help them but for me nothing but time worked.  Unlike you most of foot would get numb but it seemed to start in that toe.  The muscle that cramps on me is not the back of the calf it's the one that runs up the outside of my calf.  Very hard to get a really good stretch on it. 

I wish I was more help.  I can't say if you should stop running but if you don't think it's a cramp and you've been running very long I'd say you need to see a doctor. 

Other than that I have no ideas for you.  .