I hope your daughter continues to feel better! Your doctor is 100% right, even though it doesn't seem to "make sense" without some background information.
Anemia is a characterized by a low amount of hemoglobin, which is found in red blood cells (RBC). It's purpose is to carry oxygen, so when your levels are low, you tend to have decreased energy, because the oxygen needs of the body aren't easily taken care of by the blood. There are 3 primary ways people get anemic - loss of blood, reduced production of RBCs, and destruction of RBCs. Other reasons can include specific medical conditions, or vitamin deficiencies.
Your body makes RBCs in the bone marrow, which is also where it makes the white blood cells (WBC) that your body uses to fight disease. So when you are sick & fighting an infection (such as a virus), it can cause lowered hemoglobin & anemia, because the "factory" was processing a priority order.
Long story short, don't worry about it for the next two weeks. Depending on your daughter's age, the type of virus, how long she was fighting the infection, and how healthy she was when everything started, the effect on hemoglobin can vary. If the levels are still low, her doctor can then assess other functions to pin-point the cause, which often is due to low Vit B12 or Iron, both of which can be taken care of with diet or supplements for a period of time.
This link explains everything I have above in much more detail, & is very user-friendly to understand. Don't panic when reading it! It does list a variety of reasons (very thorough) but just because 3 items are listed doesn't mean that all 3 are as likely to happen.
http://www.news-medical.net/health/Causes-of-anemia.aspx
(I can't get it to fully hyperlink, you'll need to highlight, & then copy/paste into your browser)
T. :)