If its simple, perhaps a slipped disc of some sort, pain is usual felt from the hip to the feet, or perhaps like me earlier this year, I pulled a ham string muscle, I felt some of the pain behind my knee. Arthritis perhaps? In my case nothing worked like rest and a bit of help from a frozen bag of peas! Heat or cold, never did find out if one was better than the other, so stuck with peas!
If you decide to go to your GP ask for a visit to the physio. I did, they were very helpful, at least confirmed my own diagnosis so that made me feel a bit better straight away.