If it's on the inside bottom of your foot, where the arch of your foot is, it's called plantar fasciitis. Sometimes, it is due to a weak arch...more common with people who have flat feet...do you? An exercise you could do to strengthen the arch is called the towel gathering or towel scrunch exercise. Sitting barefoot, lay out a towel in front of you and place your foot on it. Using the toes, gather the towel towards you by flexing (or "scrunching") the toes. Do 2 or 3 towel lengths every other day, and see if this gets you results. Just remember, we are trying to strengthen the arch, so results won't be immediate...give your muscles time to build up some strength.