The foot contains fluid filled sacks, or bursa, that help protect joints against overuse and friction.  The inflammation of these fluid filled sacks is called a bursitis.  It can present as aching and swelling, irritated by activity of shoe gear.  It is important to have your foot examined to rule out any other cause of pain such as fracture of the bone or infection, and to have the diagnosis treated right away to get you out of pain.