IT'S NOT CANCER! Huuuuuuge sigh of relief.
They've diagnosed her with Juvenile Rheumatoid Arthritis. It appears to only be affecting one joint (in her knee), and I think they've caught it early enough that there's little concern for stunted growth and other nasty things. She's going to get some injections and then be put on some kind of medication, and hopefully that'll be the end of it.
Thank you girls for all the love and support. It really means a lot to me :-)