What Causes Fatigue And Long Naps After Viral Infection?

My two year, one month grand daughter had a viral infection a month ago. She has been very fatigued and is taking two long naps each day, sleeping between 17 and 19 hours a day. This is very concerning as she is no longer doing things she loved to do (dancing, playing dress up, singing). When she plays, she plays quiet things like rocking her doll in the rocking chair. She now struggles to climb stairs, which she could do easily before. She is weak and unsteady on her feet. Her doctor says all blood work is normal, and she will have a sleep study to rule out apnea. i am very concerned!