A hand holding U.S. dollar banknotes in China on January 25, 2018. Zhang Peng | LightRocket | Getty Images

The October stock market sell-off has been puzzling to many investors because most companies are riding high on business confidence and a tax-cut windfall. Plus, profit growth has been solid, and many companies have been beating expectations on strong sales numbers and a positive outlook.

But one real worry cited on post-earnings conference calls is the rising U.S. dollar, which makes business overseas more expensive for U.S. companies. The strong dollar was cited by companies as varied as Anheuser-Busch and PPG Industries. And it's a problem that is only getting worse. The gains in the U.S. Dollar Index, a measure of the greenback against a basket of major currencies, are accelerating, up another 2 percent in the last one month. By no coincidence, the S&P 500 has headed lower at the same time. 3M revised its earnings per share estimate to include a 5 cent per share hit from currency translation. It previously had factored in a gain of 10 cents a share. And United Parcel Service said currency fluctuations could be a drag of $35 million to $45 million in the fourth quarter. Add that to rising interest rates, rising fuel costs and the early effects of tariffs on imports of steel, aluminum and other manufacturing materials, and executives have struck a more cautious tone for the near term, though the overall message is still positive. The dollar was higher versus most currencies again on Friday morning as stock futures pointed to another big drop.

"The market doesn't believe 2019 growth is going to be anywhere near what it is expected to be," said Nick Raich, CEO of research firm Earnings Scout. Raich adds that there's no evidence any of these fears will come true. Estimates for 2019 first- and second-quarter earnings have actually inched up, he said. And revenue growth of 8 percent for the S&P 500 companies that have reported so far is outpacing expense growth.

'Unfavorable impact'