American Express (NYSE: AXP) has been posting solid results of late, but investors are worried about how long that can last if the economy slows ahead. AmEx, a financial services company historically known primarily for catering to the rich, has done a good job in recent years expanding its reach to a broader slice of the market and growing profits in the process. The Federal Reserve's aggressive moves to push interest rates higher threatens to slow economic growth and make it harder for consumers to pay their bills.