Louise - lie on your back and lift your leg up straight, if poss have someone else push gently backwards. If this really hurts, chances are high you have sciatica, which is an inflammation of the sciatic nerve running down your back. Not something you can cure yourself. See a physio or sports doc. Good luck for FLM.
Oh, and a little secret of mine - if your back problem turns out not to be a serious thing and you intend to get round FLM anyway, buy some Salonpas pain relieving patches and slap one on where it hurts. You can usually buy these at registration, or they can be ordered through a reputable chemist, like Boots. They really work well. A word of warning though - DON'T attempt to cover up real pain just for the sake of a race, it isn't worth it, and there will always be another race - you only have one body!