You didn't say whether arthritis runs in your family, or what kind. If rheumatoid arthritis runs in your family, she may have it, and that's waayyy beyond my knowledge, what little I have.
I had arthritis when I was twelve - I guess it was "growing pains" because it went away and didn't come back. I just suffered through it (it was when I was outside in the cold, not inside).
I've heard that glucosamine, chondroitin and Ester-C supplements can help tremendously with arthritis inflammation and pain. Fish oil supplements can reduce inflammation. You can get them in supermarkets and health food stores (I'd recommend a health store though) and they're far more affordable than those awful pharmaceutical drugs.
There's also something called an "anti-inflammatory diet" which is very strict. If she can follow the diet, she's one self-disciplined 12-year-old. Maybe she could do part of it. You can find it online by googling "anti-inflammatory diet". Good luck!