The year is ending with some pain for 2014's biggest bond winners.

After gaining 27 percent this year, the longest-term Treasuries are now falling, with yields posting the biggest two-day jump since July 2013. The notes have lost almost 2 percent since Dec. 16, the day before Federal Reserve Chair Janet Yellen suggested she may raise interest rates in the middle of next year, making comments that confused the market about whether she was hawkish or dovish.

This may mean a turbulent year lies ahead for investors who piled into longer-dated bonds as they girded for slower growth across the globe with oil prices plummeting. If the U.S. economy keeps showing signs of improvement—as it has this year—those investments may be losers.