Pain in left side, radiating across stomach and back

I have been experiencing moderate to severe pain in my left side since before Xmas. This pain is predominantly in my kidney region but it radiates up and down my left side (as low down as into my leg) and across my stomach. I sometimes get pain in my right side but never as much. I have had an ultrasound and xray to look for kidney stones but nothing was found. I do suffer from Gastro Oesphagul Reflux Disease however this is under control with Lansaprozole and Ranitidine. I have had blood tests done and have been told these are 'normal'. The pain did go but has now returned, and this seems to be a pattern. My whole left side is now very tender to touch and I am starting to worry about what it could be. Apart from this I feel perfectly healthy. Can anybody help? I don't feel as though my GP belives me now. He just sent me away and said he thinks it's my back but I know it isn't.