What Causes Throat Soreness, Muscle Aches With Joint Pain, Headaches And Fatigue?

My 16 yr old daughter starting having flu-like symptoms over 3 weeks ago. It started with a severe sore throat, muscle aches, cough, joint pain, headache & fatigue. After about a week the cough went away. All the other symptoms are hanging on. She has missed over 2 weeks of school because she is so tired & sore. Her doctor tested her for mono which came back negative, tested her thyroid and that is normal, & her blood count came back normal. She has never been sick like this before and it is disrupting her life. Her doctor told me to wait another week and if the symptoms are not gone to bring her back in. What should I do in this situation and what tests should I be asking her doctor to do? Thanks.