What Causes Pain In Left Leg When Putting Pressure On It?

Hi, I've been running extensively for about a year now and since December '09 I've been running 5 miles a day for about 5-6 days a week.In the past week my left leg has been hurting pretty bad and everytime I put some pressure on it, it hurts quite a bit.I haven't ran for the past 3 days, it still hurts.What should I do? How long do you think it will take to pass? What do you think the problem is?