(Federal Reserve Chair Janet Yellen delivering her semiannual testimony on monetary policy.Thomson Reuters)

Federal Reserve Chair Janet Yellen's increasing concern over low and falling inflation helped US stock markets hit record highs last week.

Yellen testified before Congress for two days, and Wall Street found enough concern that inflation might not in fact head back up toward the Fed's 2% target to justify renewed market bullishness.

Yellen, as she has in other statements recently, told lawmakers she expected low inflation to be transitory. "Temporary factors appear to be at work," she said. "It's premature to reach the judgment that we're not on the path to 2% inflation over the next couple of years."

Crucially for the market, however, she reiterated that "it's something we're watching very closely, considering risks around the inflation outlook."

(Bank of America-Merrill Lynch)

Thomas Simons, an economist at Jefferies, thinks a single word shift in Yellen's description of inflation speaks volumes of her potential openness to a pause in interest-rate increases, which began in December 2015 and have brought the central bank's official interest rate target to a range of 1% to 1.25% following the latest hike, in June.

"Yellen's comments before Congress this past week were significantly more dovish about inflation and rate normalization than either her June 14 press conference or the text of the Monetary Policy Report released just a few days prior to her testimony," Simons said.

"The change in Yellen's description of the recent inflation deceleration from 'idiosyncratic' to ‘unusual' was both a preview to Friday's CPI data release and a signal that the inflation picture has been elevated from an annoyance to a source of concern that is making it difficult for policymakers to come to an agreement on the detail of the Fed’s normalization efforts."

The consumer price index held steady in June, and the annual rate slipped further below the Fed's target to just 1.6%. The Fed's preferred inflation measure, the personal consumption expenditures index, also recently fell to a six-month low of 1.4%.

(Andy Kiersz/Business Insider)

"The combination of Yellen's focus and the dismal CPI data takes our expectation for a September rate hike off the table," Simons added. "A December rate hike is still on the table, but is also dependent upon the appearance of some stability on the inflation picture."

The Dallas Fed president, Robert Kaplan, seems to be edging closer to the view of Neel Kashkari, the Minneapolis Fed president who has dissented twice this year against Fed rate hikes, wanting to wait for further evidence of inflation and wage growth.