That's interesting, because during the bad step, it sometimes feels as though the bones are shifting in my foot.
Yes - I can feel the bones shift when this is happening. Usually when this happens I feel the bones shift, and they don't go back easily - so once I take that first "bad step" the remaining steps are bad too, until I manage to move those bones back to where they should be. They pop out of place because the tendons and ligaments in my foot are toast - I've broken that foot three times and sprained it more times than I can count (so, as Peppergal suggests, tendinitis is part of the problem too, as is arthritis).
The nerves that run across the top of the foot are big nerves - the dorsal medial cutaneous nerve and dorsal intermediate cutaneous nerve supply feeling to the top of the foot. Problems with those nerves tend to arise either from wearing shoes too tight or dropping something on the foot, which can crush those nerves. When my cuneiform drops, though, it puts pressure on the medial and lateral plantar nerves, and when those get pinched, it can cause the muscles at the bottom of the foot to cramp up, and cause pain that radiates out to my toes along the bottom of my foot. Now that I'm re-reading all your posts, I'm not entirely sure that what I'm describing is the same as what you're describing.
That said, I echo the suggestion to get to a medical professional, and I hope that your foot is back to a happy state soon.
