I have an extra bone in my left foot .i am flat feet. my foot hurts doing any kind of activity. hurts just sitting still..

Very common: Persons with an extra bone in their foot, very commonly are flat-footed. With chronic pain at this immediate area on the inside of your ankle, have the extra bone removed, but there is conservative care. If surgery is elected, address your flat feet as well. Go to hyprocure .Com and find local doctor.

Conservative tx: is orthotics, surgical tx is surgery. The pain when sitting still is likely because the foot was used and you are stressing the foot (i.e posterior tibial tendon)