It's been a May of mayhem for markets, and the charts suggest the might have further to fall before it finds support.

"There's probably more pain to come through the summer months," said Craig Johnson, chief market technician at Piper Jaffray, on CNBC's "Trading Nation" on Friday. "When I look at the chart of the S&P 500 ... that's not a chart that suggests that we're setting ourselves up for the next move being higher. It looks like ... some sort of head-and-shoulders top, either a double top, triple top."

The S&P 500 has fallen 4% in May, pulling back on uncertainty surrounding the U.S. and China trade talks. This weakness comes after an exceptionally strong stretch for the benchmark index — from the end of 2018 to April, the S&P 500 had rallied nearly 18%.

Following the May sell-off, the S&P 500 now faces its next critical test, says Johnson.

"I would say 2,800 is really the line in the sand between the bulls and the bears, and a break below that would suggest that we could see a pullback perhaps to about [2,650], which is about a 50% retracement from the lows we've seen in December to the April-May highs," said Johnson.

Johnson's 2,650 pullback level implies 6% downside from current levels. It would also push the S&P 500 into a correction, representing a more than 10% drop from its May 1 record.

Steve Chiavarone, portfolio manager at Federated Investors, agrees that the worst might not be over for markets.

"It's certainly possible that the sell-off here isn't done. Our view is that we're kind of stuck in the mud here where you've got some downside risk because we don't have resolution on [trade]," Chiavarone said during the same segment.

However, unlike during the worst of December's sell-off, Chiavarone says a patient Fed willing to hold on rates should put a floor underneath the market until the next catalyst arrives.

"We don't see big downside," he said. "It's hard to see big upside, though, with this level of uncertainty, so we're looking for areas that are relatively stronger. Dividend payers — they're benefiting from the Fed being on the sidelines. ... There's an opportunity for small caps which are a little bit more domestically focused and also benefit from lower rates."

The SDY S&P dividend ETF has fallen 3% this month, a shallower drop than on the S&P 500. The IWM Russell 2000 ETF, which tracks small-cap stocks, has fallen 5%.