What could cause severe bone pain on the left iliac crest?

A number of things.: It is difficult for a person to tell exactly where pain at the left iliac crest comes from. A while it could be from the bone below the hip, it can from the soft tissues: muscles/ skin/ligaments/nerves. Additionally pain can be referred: from some other structures higher up. So one needs to see the doctor for an evaluation to determine where the pain really is.