Being flat footed can be caused by several different things.  Some of the causes are foot injuries, illnesses, and rheumatoid arthritis or arthritis in general.  There are also studies done that have attributed adult flat footedness to going barefoot often as children.  If you have rheumatoid arthritis you should bring this to your rheumatologists attention.  I have severe damage in my feet due to RA and now have what is called "fallen arch" where my feet are now flat due to damage.  It can be quite painful at times.  Now if you don't have RA, you should speak to your primary physician about this.  He will want to know if you have always been flat footed, if it causes pain, and may ask you some more questions.  If you have always been flat footed it is most likely in your genes, if not you owe it to yourself to get it checked out to make sure you don't have any damage.  Hope this helps.  Good luck and best wishes!