What Causes Tiredness And Sinus Infection In A Child?

My 9 year old (very active daughter) has been so so so tired lately. It is hard to get her going in the morning and throughout the day. She has been sick with a sinus infection and terrible cough for over a month now. She is negative for Mononucleosis. She started an antibiotic 4 days ago and that seems to be helping her sinuses and congestion but she is still exhausted. She doesn t even have energy to go to gymnastics! And that is her favorite thing in the world! Should I have the doctor run more tests? I wonder if she may be anemic.