for years it was a mystery disease/allergy for my sister Aline, she's always had CFAA (cystic fibrosis associated arthritis) the worst. It started with red spots everywhere and one of her joints swelling up and hurting... along with fever, aches & pains. She has suffered from it since she was a pre-teen.
the children CF clinic never knew what it was, she was tested for everything. the first day I was seen by my adult CF doc, I asked him about it and he said it was CFAA, pretty common in CFers.
I googled it, and here are some good webpages I found.
it explained everything that had been going on with my sister and as I got to be an older teen me as well... we found a good natural supplement for muscle recovery that keeps away the symptoms. if I miss taking them for a while I get a huge CFAA attack... so, I try to remember to take them!
losing sleep and a low immune system also contribute to an attack, today I got red spots and slight aches & pains. if it gets bad enough I take advil, it helps with the inflammation. the reason I believe this happened was the loss of sleep these last couple days and I may have forgotten some of my pills (OOPS).