I'm not sure if your aware but you've posted your question in the MS community, this link may help in explaining this to you.......
http://www.foot-pain-explored.com/foot-cramps.html
Cheers...........JJ
As JJ mentioned, we are an MS focused bunch :-)
If you are diagnosed with MS, or suspect it for other reasons, wha you describe can be spasticity, neuropathic pain or a combination of the two. Both can be MS related.
In any case I would suggest that you see your doctor.
Kyle
I'll reiterate what Kyle and JJ said. Go see your doctor if this continues. He/she can run some blood tests to ensure you're not deficient minerals or vitamins ie: Magnesium, Calcium, etc.
If you're diagnosed with MS as Kyle says...etc.
Good luck
Lisa