Janet Yellen just made an unsettling admission about the economy

It didn't take long for Janet Yellen to rid investors of that rare feeling of predictability from the Fed.

"The data have not notably strengthened," the Federal Reserve Chair said during her March 15 press conference after the central bank raised interest rates for just the third time since the financial crisis.

This hike was one of the least surprising to markets, with traders pricing in a full 100% chance that it would take place. The Fed chair just killed that kind of confidence for any move going forward.

Yellen was talking about the lack of economic progress since the Fed's last meeting in January. She went further though, saying that the Fed doesn't see any evidence that the optimism of a record-breaking stock market has made its way into spending by companies or people.

Investors are desperately seeking clarity on the timing of future rate increases, and they will now be left to speculate as to whether May, June or September will bring the next move. Much hangs on US President Donald Trump's ability to push through a promised agenda of tax cuts and infrastructure spending, which has fueled Wall Street benchmark's higher. The early reception of the Trump/House Republican healthcare plan is hardly encouraging.

The Fed has a lot of wiggle room, though. US inflation continues to run below its target and is expected to do so for the foreseeable future. Another reason not to rush the next hike: the Atlanta Fed's GDPNow indicator has just slipped rather sharply in a matter of weeks from 2.5% for the first quarter to just 0.9%.