The yield on the German 2-year bond dipped briefly into negative territory on Thursday before getting back above zero percent. Essentially, investors holding short-term German government bonds (also known as bunds) would rather get no return, or even pay, in order to hold bonds rather than cash.

According to Gina Sanchez, founder of Chantico Global, that reflects two major worries in the market right now.

"Europe has been in a very weak recovery – more like stagnation – and they've been battling deflation issues," said Sanchez, a CNBC contributor. "But what's happening in Ukraine and Europe's response to Russia is also important because that could actually have a big impact on Europe."

(Watch: European shares close lower after ECB decision; Russia weighs)

Earlier in the week, Russia banned imports from the European Union, Norway, the United States, Australiaand Canada. This was done in retaliation for those countries' sanctions on Russia in the wake of increased violence between the Ukrainian government and Russia-backed separatists.

"The 2-year dipping into [negative] territory… is reflecting some of those concerns," she said.

Mark Newton, chief technical analyst at Greywolf Execution Partners, agrees that what's going on in Europe may signal trouble for the United States – at least in the short-term.

"All of this fragility over in Europe is slowly but surely spreading contagion to the U.S.," Newton said. "Slowly but surely, we are seeing signs now that it is starting to affect U.S stocks as well."

(Watch: US stocks end lower; investors rattled by Russia)

For the technician, that was evident when the S&P 500 broke below its July lows earlier this week. "As of yet, we haven't really reached areas of support," he said. "Momentum is sloped negatively; we're not really that oversold yet, and we're still in a downtrend on a short-term basis."

However, a longer-term chart of the S&P 500 shows this is a short-term correction in a larger uptrend in place since 2013, according to Newton. He also sees support in an area between 1,885 and 1,895. "On a pullback over the next week or two, [that area] should be an area to buy from a trading perspective," he said.

"Europe is been far weaker than the U.S.," he pointed out.

To see the full discussion on the effects of Europe on U.S. stocks, with Sanchez on the fundamentals and Newton on the technicals, watch the above video.

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