With 69 straight months of job growth, unemployment at a seven-year low, the Dow Jones Industrial Average near all-time highs and the Federal Reserve confident enough to hike interest rates, you might think a sturdy U.S. economy has given everyone reason to cheer.

Apparently not everybody. New data from Wells Fargo indicates that something is spooking a sector of the U.S. population - the nation's seniors - in a serious way.

On its Investor and Retirement Optimism Index, which tracks investor sentiment, retiree optimism dropped 23 points in a single quarter.

"It was a very big drop, and the first time in over a year that retirees were less optimistic than non-retirees," says Zar Toolan, director of advice quality for Wells Fargo Advisors.

Over the same three months, non-retiree investor confidence actually went up by 10 points.

Michael Sokal, a 70-year-old retired teacher in Worcester, Massachusetts, is among the worriers.

"Our portfolio isn't what it was when we first retired," Sokal says. "We have been drawing it down, and the performance isn't what we hoped for. It's a big concern."

When Wells Fargo analysts started digging a little deeper, they discovered a few different factors at work.

Costs Rise, Investments Flat

With the cash in their bank accounts earning virtually nothing, and the stock market essentially flat for the year, retirees have not been seeing any portfolio improvements. Inflation, a notorious portfolio-killer, looms whenever an economy is heating up.