Earnings and guidance disappointed, but the pullback could be an opportunity.Shares of pain-management equipment specialist Zynex (ZYXI -12.37%) were falling in Friday trading, down as much as 23.4%, before recovering to a 13.4% decline as of 3:25 p.m. EDT.The medical equipment company reported earnings last night that, on a headline basis, disappointed mightily. But there were actually some good reasons for the soft numbers, making the pullback in this small-cap stock a potential buying opportunity. Earnin ...