The stock market was heading for some big gains—until Federal Reserve Chair Jerome Powell stopped it in its tracks.

The Dow Jones Industrial Average fell 131 points, or 0.4%, while the S&P 500 slipped 0.2% and the Nasdaq Composite was little changed. The Nasdaq was up as much as 3%, while the S&P 500 was up around 2% immediately after the announcement. The Dow was also in the green earlier in the day.

The…