I will also suggest Lyme Disease as it can cause knee pain, itchiness, and even B12 deficiency.
I don't think it will be celiac disease, when you eat gluten and wheat it go's right through you and stops the vitamins you take from being absorbed through the intestine which makes you really ill and run down and makes you lose a lot of weight. I think if it was hepatitis you would have a rash because its to do with the liver and if your liver is struggling you get a really itchy rash.It could just be she has had a bad reaction to something she has ate or a really bad infection. I would go see your doctor and let them do tests before you start getting to worried.
Itching without a rash, and loss of appetite needs an evaluation to rule out hepatitis.Do get her urine tested for bile salts and pigment.ok?
She should be tested for lyme disease.
My entire family has lyme and we all have the itching sensation along with joint pain, muscle pain, headaches, neck tightness and many more symptoms. Lyme effects the nervous system and can cause these conditions. No bulls eye rash required to have lyme. You can also show up negative on testing and still have it. The testing is very poor for lyme disease so the best way to be sure is to see a lyme specialist. This is something to consider as a possibility.
Have her tested for Celiac disease. It can cause vitamin deficiencies and skin issues, including itching.
Here's another thread here dealing with the same problem - the consensus seems to be B12 deficiency.
Lots of good ideas.
http://www.medhelp.org/posts/Dermatology/Extreme-Itching-all-over-body-w-NO-rash--welts-or-hives/show/6426