Until Friday, September had defied worriers. The stock market was rebounding from an August swoon. But Washington’s budget fight jolted investors, reminding them that the next few weeks could bring a lot of uncertainty. Wall Street hates uncertainty. Stocks fell in a sell-off that wiped out most of the gains from a rally after the Fed said it would keep its stimulus program intact. Friday, the US House voted to defund the president’s health care law. The vote wasn’t a surprise, but it reminded investors that the House and Senate are poised for a showdown over federal spending, with a government shutdown possible on Oct. 1.