Well, it could be a number of things. Do you run or walk frequently? Also, do you have a job that requires you to be on your feet for most of the day?

It could be tendinitis or, in a worst case scenario, stress fracture. Try icing it down for a few days, and try taking an anti-inflammatory (aspirin). If that doesn't help, i'd go see a foot doctor (podiatrist) or your general check up doctor for proper treatment. Wish i could help more. Take care.