Chronic back pain for many years.

Patient: Hi im 23 now and iv had lumber pain since i was 16 i think. sometimes its really severe and causes me to lay in bed but most of the time its manageable but still there. i get alot of exercise as im at work for 8 hours and on my feet for 7 hours and constantly walking around although when i stand still for more than an hour my back starts to really hurt mainly the lumbar and under my shoulder blades. iv had x rays taken and the doctor said everything was fine but referred me to a physiotherapist, who told me i would always have this problem but didnt diagnose anything. so what can it be?