Without seeing the rash it is difficult to determine how it fits into this picture but a WBC of 2.8 is certainly low and inconsistent with strep. A low WBC can occur with viral illness, a much more likely situation than other possibilities for low count in her age group. The combination low WBC and rash suggests viral infection. A much more remote possibility is early leukemia which should be identified quickly as it can be readily treated. Autoimmune disorders can also cause a low WBC but this, too, is far less likely at her age. My recommendation is that you have her primary care doctor see her tomorrow, consider rechecking the blood work with a bit more detail, and follow his or her advice regarding continued monitoring until the counts return to normal vs more in depth investigation.