M.R.
My first thought was to take her to the dentist, and see if she is grinding her teeth. I do this, have done it since I was a kid, and it causes head aches and problems sleeping. Each time I go they say...reduce my stress. Easier said than done. Any way, anxiety could be causing all of it (event he grinding,if that is what it is) but the aches and pains are real. Take her to see a Board Certified Child Psychiatrist, because there is help for her. Some kids have "somatic" complaints (not real) when they are anxious, and some feel pain because depression can be caused by anxiety disorders, and the pain people feel with depression is very, very real. Some others, do things, like teeth grinding or clenching other muscles because of anxiety that cause real pain. Teeth grinding is one that just happens to become more of a problem all by itiself, so even if you treat the underlying anxiety or depression, once you start grinding, it may not stop.
M.