This could be due to a disc that is bulging or any number of
things. I think your physio did the right
thing in ordering an MRI
etc so that you'll know for sure. I would in the mean time, however, avoid prolonged standing activities and practice positions where your back is in a more "open" posture, for example, lying on your back with your knees and hips bent and hugging your knees towards your chest. This should take some of the pressure off the nerves in your back.