PLEASE HELP lower back,hamstring and knee pain

Please can someone give me a quick reply for this, I have pain in my lower back and right ass cheek, the pain also shoots dwn my hamstring and sometimes into my knee :( i have had it for a week now but until yesterday the pain was getting worst now it has eased slightly. I can walk with no pain and even jump vertically with no pain. but i cannot sprint or even jog and tomorrow i ave the finals of an athletics competition as i am a sprinter, i hope someone can help me as soon as possible

thanks.