What causes severe back pain on trying to stand up?
Low Back Pain when arising from sitting to standing is usually caused by a problem effect the posterior column of the lumbar spine such as arthritis, fracture, stenosis, spondylolithesis, and more. It could also be caused by nerve impingement or sciatica, or a number of other conditions. Only a trip to the doctor for a proper examination can give you the correct diagnosis for your pain.
Here are some links on back pain that you might find useful:
Sciatica – An Overview
Disc Herniation: What is It?
Dr. Christina Lasich, MD