Lower energy prices may be on the horizon, and energy investors aren't happy about it.Starting the week off on a bullish note, both the S&P 500 and Dow Jones Industrial Average are trending higher today. The same can't be said for shares of upstream energy leader ConocoPhillips (COP -2.80%). While there's no direct news from -- or about -- the company that speaks to the stock's decline today, investors are simply responding to tumbling energy prices.As of 2:56 p.m. ET on Monday, shares of ConocoPhillips are ...