It's possible that what you are experiencing is something called plantar fasciitis. Plantar fasciitis is a common condition in which the connective tissue on the bottom of the foot near the heel bone becomes inflamed. The causes of plantar fasciitis are not clear, but they might relate to repetitive trauma or strain on the tissue (from walking, running, etc). The primary reason why I think this might be going on is that there is a strong association between plantar fasciitis and the type of shoe you wear, with shoes without arch support causing the most symptoms.
Of course, if you have a rash or sore on the bottom of your foot, or if you are diabetic or have another complex chronic medical condition, then there could be a number of other possible causes that your should have evaluated by your primary care doctor
On the other hand, if it is plantar fasciitis, then often simple stretching exercises can be quite helpful. You can find these on the internet or talk to your doctor. Also wearing supportive shoes with a good built in arch or even an arch insert can be very helpful. Antiinflammatory medications may help temporarily to relieve the pain.