Have it evaluated. Have your feet evaluated by a professional and get the appropriate treatment. You may benefit with use of custom foot orthoces.
An new flat foot. Foot dhape varies many painless feet have no arch if this is a new painful flatfoot see a orthopedic foot and ankle specialist dr hanson in seattle is the best my opinion if you have had flat feet since childhood and are painless love who you are painless feet are a gift not all flat feet are pathologic the army stopped kicking solidiers out for them years ago as many flat feet perfotm well if.
Relax. Flat feet may or may not be anything to worry about. If you've always had flat feet, you leave it alone. You don't need anything except maybe using wide comfortable shoes. If you are getting worsening flat foot, especially associated with pain, you should be evaluated by a foot and ankle specialist.