Hello Aussie mate,
Its hard to say what this problem might be without knowing your age, what treatment you had for your L5 region was and what your description of feeling better is...
however, some of the paraverterbal muscles (the long ones) flow down and attach to areas of the pelvis..if there is a tightness there , you might experience in your thorax, shoulders and neck as well...
it sounds to be like a postural problem...certain apsects of your daily activities may be contributing to this but its hard to say from your description really..
cheers