There could be a number of possible causes. First of all, is there any chance you've injured it over the past couple of weeks? (apologies for stating the obvious). If so, you could have broken or chipped a bone. The next obvious is footwear: Are your shoes etc fitting OK?
If you can eliminate these, there are a few other possibilities. If you're diabetic it could be a type of peripheral neuropathy associated with the disease. If not, you can get something similar called peripheral vascular disease which is very common in extremities especially at this time of the year. Too much of the old vino can also bring on another version of the disease called alcoholic neuropathy (and no, you don't have to be an alcoholic to get it - just a regular drinker).
It's also possible to have a type of Raynaud's Phenomenon that will give you these symptoms. Vasculitis is another possibility as are certain types of sciatica. Apart from these, the only other reasonable possibility I can think of tonight is frostbite, which is pretty unlikely unless you've been skiing etc recently.
Incidentally, if you smoke, this could well make the situation worse if you have any type of neuropathy.