Question
I have a 6yo Daughter, in competitive gymnastics. She has not been able to bend backwards in 6 weeks. We cant pinpoint any action that caused the injury. MRI and x-ray were negative. She experiences the pain in her lower left side of her back just above the hip bone. She is not bothered by running jumping or any other normal 6yo actions just the bending backwards. Looking I would say she begins to feel pain at about 10-20 degrees when bending back. The pain was first felt Feb 9th.

Answer
My concern would be what is called a spondylolysis, which is a stress fracture due to overstress on the lower back. This is found on the  MRI as edema to the injured area. However, you said the MRI was negative. So, her back pain is likely other joints or muscles. It sounds like you may be describing the sacroiliac joint. I see patients like your daughter and they respond quite well to chiropractic adjustments and it is safe. We also perform some therapy such as therapeutic laser or electrical stim to the region. When the back is really flared up, you have to back off the gymnastics for a week or so.  I would suggest you see a well known Chiropractor in you area for an exam.