What Causes Lower Back Pain Despite A Normal X-ray Report?

I need some advice. My 7 yr old daughter was complaining about lower spine pain. Last year, her pediatrician noticed a curvature in her spine. We had several x-rays for scoliosis, all normal. I would say about a week and a half ago, she was complaining about spine pain. It was on my radar, so I was going to schedule a second opinion to get a second set of x-rays. Then she was crying at recess on Monday due to spine and joint pain. She wound up in the school nurse s office. Long story short, I took her to her pediatrician and she was diagnosed with Strep- presenting with reactive arthritis. Has anyone else experienced this? She showed absolutely no symptoms of strep until the joint pain. Now I m worried about Rheumatic Fever, Any advice would be very helpful and welcome