I had mono 2 years ago, You get really tired and feeling unwell in a wierd way. I developed Anxiety after having this disease, still not well. I hope you find out what's wrong with your daughter, not knowing the problem is the hard part, not knowing how to fix it or how to work trough it.
as a parent not knowing is the hardest part, how you can help you're daughter, be patient, listen to you're daughter, If its mono it might go over and she might not develope other things afterwards.
I hope you get answers for you're daughter :)
Has she had her folate (folic acid) and thyroid level checked? It doesn't sound viral to me. The large red cells can be caused by loss of blood where there is anemia, hypothyroidism, liver problems, folate deficiency and some side effects of drugs for seizures, cancer and autoimmune disease. So if you haven't get had the blood tests for thyroid, folate, liver enzymes and hemoglobin (complete blood count) I would have those taken.
My freshman year of high school i caught mononucleosis (mono) from a friend sharing drinks and other things. My symptoms were like the Flu was how my doctor described it. I was always tired and many aches including bad headaches. After a while I had the chills and a slight fever with a sore throat along with the other symptoms.. If your daughter went to the doctor and the doctor did not say anything about it i would go back and ask. My doctor to told me he could not find anything wrong with me until I was informed my friend in fact had mono I then went back and my doctor then told me that was the cause of my problem.