What could cause lower back pain while fighting off a viral infection in an elven year old?

Virus caused: Viruses can cause painful muscles (myalgias) as part of the illness. Typically these are short lived, lasting only a few days. They respond to fluids and children's ibuprofen.

Muscular: Muscle symptoms are fairly common with flu-like symptoms due to viral causes and respond to symptomatic treatment like rest, heat and otc medication.