Yellen says it wasn’t the Fed’s credibility, but rather the data, which led the Fed to hike rates. Recall that at the last meeting, in October, the Fed raised the stakes by saying a hike would be considered at the next (e.g. this) meeting. So some skeptics said the Fed boxed itself into a corner.
Since then, the U.S. economic data has been mixed — pretty good news on the jobs front, very decent numbers on the services side, and frankly awful data from manufacturing.
Yellen points out that the impact from rate hike has a lagged impact. The Fed is aiming for the economy nine months from now.