What is causing my back to ache?

I have been suffering with a low back ache since i was 6-7 years old. I am now 30. The ache is constantly there. Since I have had my child (in 2004) I have been experiencing what some people have described as sciatica. I have the back ache, and now and again, I will get a sharp pain in my ock. This usually happens when I am stood for a while. The back ache never seems to go. My GP said for me to contact Physio Direct. What could be causing this long term cronic back ache?