What Causes Pain In Legs Since The Age Of 6?

My daughter is 21 and has suffered since about the age of 6 with leg pains. She has had many tests - muscle biopsy, blood tests, x-rays but nothing conclusive shows up - she does have some raised levels of something which showed up in a blood test. She has been so distressed lately with the pain, has to take paracetamol every day and I'm worried this is not good. She has a tens machine also for the pain. She is otherwise fit and healthy - we really don't know what to do next - any help please