Is a quarter (US 25 cent piece) mint-marked "S" a counterfeit?

Got it in a change machine at the laundromat. Very shiny, lighter than other quarters. Mint stamp is not D nor P, but S. Illinois, 2003. Edges rather sharp.
Update: The state series of quarters have always been marked D or P for Denver or Philadelphina mints. This seems very unusual for have an S, perhaps for the San Francisco mint.
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