@ocelot60- I think that you do have a sprained foot. Even though you didn't fall or do anything that you think may have caused you to sprain your foot, you probably did and just didn't notice it at the time. Sometimes, walking on coarse gravel or stepping on an object while walking briskly can cause a mild sprain with pain that sets in later.
I think you should take it easy for several days and stay off of your feet as much as possible. Try using a cold compress, and take some over-the-counter pain relievers if necessary. If this plan of action doesn't help relieve your symptoms of foot pain, you should see your doctor for a diagnosis.