I think you should go to see your doctor
about this. It sounds like it has been going on for quite a while, and you should have your doctor look at your foot to help you figure out what is going on. Most likely, it is not a serious medical problem, but it should be diagnosed.
One possibility is that you have flat arches. When the arches of the feet are flat, there is little support especially when standing for long periods of time. This can lead to fatigue and cramping pain, but, fortunately, this can usually be fixed simply by wearing the right pair of shoes.
Another possibility would be plantar fasciitis. This is an inflammation / over use injury
of the connective tissue on the bottom of the foot in front of the heel bone. Usually there is pain when pressing right over this area. Plantar fasciitis often responds to proper footwear. There are also a number of stretching and strengthening exercises which your doctor can give to you which will help.
In the mean time, while you are waiting for your appointment with your primary care doctor
, stick to comfortable shoes with good support (like athletic shoes) to minimize your discomfort.