What Causes Pain From Buttock To Ankle?

Hi. I am a fairly healthy 40 year old male. I was obese in 2010 and have been working out. I have lost lots of weight through diet and exercise and feel great. I am down to 180 lbs. Last night I went to the store and while walking I had SUDDEN sharp pain in my right buttock that shot down my right leg to my right ankle. It is the worst pain I have ever felt in my life. I went to sleep last night and cannot even walk on it today. Should I go to the ER or wait til monday and go see my doctor.