If the pain in your foot is only occurring when you are walking, you may want to try different pair of shoes that provide appropriate support. If you feel that shoes are not the problem, it's possible that you have strained a ligament or pulled a muscle.
I know from personal experience that ill-fitting shoes cause blisters and foot pain. Also, wearing sneakers that have too many miles on them will cause pain since there is not enough support to buffer the stress on the foot.
Improperly fit or old shoes tend to make walking uncomfortable and sometimes painful much as a bad mattress can result in back pain. Plantar fasciitis, a shortening of tissue in the bottom of the foot, usually finds you in daily pain that can ebb and flow, but never 'heals' completely.
It could be a sprained ankle. Poor circulation in the foot could could also be a cause. An unknown blister from excessive walking or running could be affecting the area also.