At a glance, Lululemon Athletica Inc. and Krispy Kreme Doughnuts Inc. seem about as far apart on the retail-product spectrum as any shopper could imagine. But their one key shared trait – an almost cult-like popularity among consumers that, as Krispy Kreme discovered, doesn’t last forever – may be starting to make Lululemon’s investors increasingly uncomfortable about the long-term sustainability of its stock price.